Under the Treasury regulations, a tax return can have more than one tax return preparer. In such case, you have a "signing tax return preparer" and at least one "nonsigning tax return preparer." The signing tax return preparer must provide his or her PTIN in the return. However, as a practical matter, even a non-signing preparer must
have a PTIN.
The Treasury regulation at 26 CFR sec. 1.6695-1(c) provides (in part):
(c) Failure to furnish identifying number. (1) A person who is a signing tax return preparer as described in §301.7701-15(b)(1) of this chapter of any return of tax under the Code or claim for refund of tax under the Code, and who fails to satisfy the requirement of section 6109(a)(4) and §1.6109-2(a) to furnish one or more identifying numbers of signing tax return preparers or persons employing the signing tax return preparer (or with which the signing tax return preparer is associated) on a return or claim for refund after it is completed and before it is presented to the taxpayer (or nontaxable entity) for signature shall be subject to a penalty of $50 for each failure, with a maximum of $25,000 per person imposed with respect to each calendar year, unless it is shown that the failure is due to reasonable cause and not due to willful neglect.
(emphasis added).
The Treasury regulation at 26 CFR section 301.7701-15 provides (in part):
(a) In general. A tax return preparer is any person who prepares for compensation, or who employs one or more persons to prepare for compensation, all or a substantial portion of any return of tax or any claim for refund of tax under the Internal Revenue Code (Code).
(b) Definitions—(1) Signing tax return preparer. A signing tax return preparer is the individual tax return preparer who has the primary responsibility for the overall substantive accuracy of the preparation of such return or claim for refund.
(2) Nonsigning tax return preparer—(i) In general. A nonsigning tax return preparer is any tax return preparer who is not a signing tax return preparer but who prepares all or a substantial portion of a return or claim for refund within the meaning of paragraph (b)(3) of this section with respect to events that have occurred at the time the advice is rendered. In determining whether an individual is a nonsigning tax return preparer, time spent on advice that is given after events have occurred that represents less than 5 percent of the aggregate time incurred by such individual with respect to the position(s) giving rise to the understatement shall not be taken into account. Notwithstanding the preceding sentence, time spent on advice before the events have occurred will be taken into account if all facts and circumstances show that the position(s) giving rise to the understatement is primarily attributable to the advice, the advice was substantially given before events occurred primarily to avoid treating the person giving the advice as a tax return preparer, and the advice given before events occurred was confirmed after events had occurred for purposes of preparing a tax return. Examples of nonsigning tax return preparers are tax return preparers who provide advice (written or oral) to a taxpayer (or to another tax return preparer) when that advice leads to a position or entry that constitutes a substantial portion of the return within the meaning of paragraph (b)(3) of this section.
[ . . .]
(3) Substantial portion. (i) Only a person who prepares all or a substantial portion of a return or claim for refund shall be considered to be a tax return preparer of the return or claim for refund. A person who renders tax advice on a position that is directly relevant to the determination of the existence, characterization, or amount of an entry on a return or claim for refund will be regarded as having prepared that entry. Whether a schedule, entry, or other portion of a return or claim for refund is a substantial portion is determined based upon whether the person knows or reasonably should know that the tax attributable to the schedule, entry, or other portion of a return or claim for refund is a substantial portion of the tax required to be shown on the return or claim for refund. A single tax entry may constitute a substantial portion of the tax required to be shown on a return. Factors to consider in determining whether a schedule, entry, or other portion of a return or claim for refund is a substantial portion include but are not limited to—
(A) the size and complexity of the item relative to the taxpayer's gross income; and
(B) the size of the understatement attributable to the item compared to the taxpayer's reported tax liability. [ . . .]
(emphasis added).