As a TPA we have been asked to commingle assets of a 403(B) and a Money Purchase Pension and recordkeep as one plan. We have the ability to establish separate sources of money for reporting purposes. However, we are not sure if we can legally hold assets for both plans under one custodial account.

PLR 9540061 discusses this

Here's a copy of the ruling Everett mentioned, in case you need it:

Thanks for the info! It was just what we needed.

You have to remember that if you want to follow PLR 9540061, you have two hurdles to jump. First, unless the sponsor is eligible to have a retirement income account under 403(B)(9), the structure will have to be Trust A under qualified plan, with Custodial Account A, Custodial Account B, and comingled Custodial Account AB. Also, this does not make it a single plan for administration purposes, including 5500, audit, etc.