A good primer on multi-state practice of law appears on How Mr. Phillips Handles Cases Across the USA. More details are found in this Legal Brief, below.
Model Rule 5.5(c)(2)
"Rule 5.5: Unauthorized Practice of Law; Multijurisdictional Practice of Law
"(c) A lawyer admitted in another United States jurisdiction, and not disbarred or suspended from practice in any jurisdiction, may provide legal services on a temporary basis in this jurisdiction that:
"(2) are in or reasonably related to a pending or potential proceeding before a tribunal in this or another jurisdiction, if the lawyer, or a person the lawyer is assisting, is authorized by law or order to appear in such proceeding or reasonably expects to be so authorized."
Purpose of Model Rule 5.5(c)(2)
"This subsection permits temporary practice in a host state under two circumstances. First, it authorizes an out-of-state lawyer’s services in the host state that are in anticipation of litigation that the lawyer expects to be filed in the lawyer’s home state. For example, prior to filing suit, the lawyer might need to go into other states to investigate, gather evidence, interview witnesses, review documents, or negotiate. Second, subsection 5.5(c)(2) permits legal services that are ancillary to litigation that is already pending either in the lawyer’s home state or in a state where the lawyer has been or reasonably expects to be admitted pro hac vice to participate in the litigation. Thus, a lawyer who is properly representing a client in litigation may travel outside the state where litigation is pending in order to gather evidence, take or defend depositions, or conduct settlement negotiations.
"Subsection 5.5(c)(2) provides the flexibility necessary for the lawyer to do all legal work necessary—wherever that work arises—to fully represent the client’s interests in pending or prospective litigation. This subsection strikes an appropriate balance between the host states’ interests and the needs of lawyers to have the flexibility necessary to fully and efficiently represent their clients’ interests in litigation. It would be expensive and inefficient to require lawyers to hire local counsel every time they had to travel across state lines to do work ancillary to litigation. This is especially true in the context of preliminary work done in anticipation of litigation that has not yet been filed. Moreover, the host state’s regulatory interest is not seriously threatened by permitting cross-border legal work that is ancillary to pending litigation because the court in which the litigation is pending is empowered to regulate the conduct of the lawyers appearing in proceedings before it. Nor is the host state’s regulatory interest seriously threatened by permitting cross-border work that is ancillary to anticipated litigation. Under Rule 8.5, the licensing state retains power to discipline a lawyer who goes out of state to do work in anticipation of litigation.86 In addition, under amended Rule 8.5, the host state has regulatory power over an out-of-state lawyer performing legal services in that state in anticipation of litigation.87 Thus, even if a suit ultimately is not filed, the host state’s regulatory interests are still protected." (Cynthia L. Fountaine, Have License, Will Travel: An Analysis of the New ABA Multijurisdictional Practice Rules, Washington University Law Review, Vol. 81, Issue 3 (January 2003), pp. 749-750.)
Variations on the Model Rules
Pro haec vice admission is specifically provided for. Subdivision (a)(1) of Section 520.11 (Admission Pro Hac Vice) of Part 520 - rules of the Court of Appeals for the Admission of Attorneys and Counselors at Law states that "[a]n attorney and counselor-at-law or the equivalent, who is a member in good standing of the bar of another state, territory, district or foreign country may be admitted pro hac vice ... in the discretion of any court of record, to participate in any matter in which the attorney is employed." http://www.nycourts.gov/ctapps/520rules10.htm#11
The out-of-state lawyer must associated a NY attorney. (Section 520.11, subd. (c).) The out-of-state lawyer is required to comply with the standards of professional conduct imposed on members of the NY bar, and is subject to the jurisdiction of NY courts as to any acts.
New York's rules governing attorneys otherwise do not mention multi-state practice of law. The state did not adopt rule 5.5 of the ABA Model Rules. See also:
FAQ - http://www.nycourts.gov/ctapps/admattrnyfaq.htm
Rules of Professional Conduct - http://www.nycourts.gov/rules/jointappellate/NY-Rules-Prof-Conduct-1200.pdf
Joint Appellate Rules Governing Attorneys - http://www.nycourts.gov/rules/jointappellate/
Texas has not adopted the Model Rule 5.5. (See Texas Disciplinary Rule of Professional Conduct, pp. 80-81.)
"The Supreme Court of Texas has not adopted Model Rule 5.5(c)(3). Although Texas has a broad definition of the practice of law, currently no Texas court decision or rule speaks directly to the question of whether an out-of-state attorney may temporarily represent a party in a private arbitration in Texas. Historically, the Supreme Court of Texas Unauthorized Practice of Law Committee, which is charged with preventing the unauthorized practice of law in Texas, has not challenged as the unauthorized practice of law, the representation of a party in a private arbitration in Texas by an attorney authorized to practice in another jurisdiction. (Footnote 56: Telephone interview with Leland C. de la Garza, Chair, Supreme Court of Texas Unauthorized Practice of Law Committee (Dec. 1, 2009)." (Bruce E. Meyerson, AAA HANDBOOK ON ARBITRATION PRACTICE, CH 13, AN UPDATE ON MULTIJURISDICTIONAL, PRACTICE AND ADR.)
"Under the Texas Disciplinary Rules of Professional Conduct, a Texas lawyer may practice law as a member of a law firm with lawyers who are licensed only in jurisdictions other than Texas and who practice law in offices of the law firm located outside of Texas. The Texas lawyer does not improperly assist in the unauthorized practice of law when non-Texas lawyers, who are members of the law firm duly licensed in another jurisdiction and who normally practice in offices of the law firm outside of Texas, from time to time provide, in compliance with any applicable local rules and without themselves establishing a systematic and continuous presence in Texas, legal services in Texas as members of the law firm." (Texas Center for Legal Ethics, Opinion 597 (May 2010).)