N.Y. Comp. Codes R. & Regs. tit. 8, § 30-1.9 – Interdisciplinary assignments.

(a) Except as otherwise provided in subdivision (b) of this section, a board of education or a board of cooperative educational services shall appoint and assign a professional educator in such a manner that he shall devote a substantial portion of his time throughout the probationary period in at least one designated tenure area except that a professional educator who teaches in an experimental program as defined in section 30-1.1 of this Subpart and who does not devote 40 percent or more of his time to service in any one tenure area may be appointed to a tenure area for which he holds the proper certification.
(b) Except as otherwise provided in section 30-1.2(b) of this Subpart, a board of education shall appoint and assign a professional educator in such a manner that he shall devote a substantial portion of his time in at least one designated tenure area except that a professional educator appointed or assigned on or after May 1, 2009 to duties described in either paragraph (1) or (2) of this subdivision, shall be appointed to a tenure area for which he holds the proper certification:
1. (1) a professional educator appointed or assigned to devote a substantial portion of his time to the performance of duties in instructional support services; or
2. (2) a professional educator appointed or assigned to devote a substantial portion of his time to a combination of duties in instructional support services and time in at least one designated tenure area identified in this Subpart.
(c) No professional educator, whether on tenure or in probationary status, may be assigned to devote a substantial portion of his time in a tenure area other than that in which he has acquired tenure or is in probationary status, without his prior written consent.
(d) If a professional educator possesses certification appropriate to more than a single tenure area and the board of education or board of cooperative educational services proposes at the time of initial appointment to assign such individual in such a manner that he will devote a substantial portion of his time during each of the school years constituting the probationary period in more than one of the tenure areas established by this Subpart, the board shall in its resolution of appointment designate such tenure area and shall thereafter separately confer or deny tenure to such individual in the manner prescribed by statute in each designated tenure area, except that individuals accruing tenure and/or seniority rights in their previous tenure area for the performance of duties in instructional support services or computer science as provided for in sections 30-1.2(c)(1)(i) and (d)(1)(i) of this Subpart as applicable, shall only accrue tenure and/or seniority rights in their previous tenure area and not in one of the instructional support services tenure area or the computer science tenure area described in section 30-1.8(a) and (e) of this Subpart as applicable.
(e) Where a board of education or board of cooperative educational services proposes to assign a professional educator having tenure or in probationary status in a tenure area created by this Subpart in such a manner that he will devote a substantial portion of his time in a tenure area to which he has not previously been appointed, the board shall prior to such assignment confer a probationary appointment in accordance with section 30-1.3 of this Subpart, designating such additional tenure area. Thereafter, the board shall separately confer or deny tenure to such individual in the designated tenure area in the manner prescribed by statute.
(f) Conferral or denial of tenure at the conclusion of the probationary period or termination of services of a professional educator during the probationary period shall affect only the tenure area under consideration.