As usual, Luke is "spot on!" I do a reasonable number of church plans and have not to date done a 403(b)(9). While I acknowledge the 'No required document" material cited, I always do documents. How else does one know what the rules are? And I work and live in litigious California so I think it is good housekeeping.
One of the details/exceptions is whether there are minor children?
I was taught that if there was minor children between the two, then the two plans are a control group.
Limit determinations are found in the regs.
most limits are beginning of the year e.g. 1.401(a)(17)-1(b)(3)(ii) "...for the calendar year in which the plan year begins."
The 415 limit is the only limit that is based on the end of the year, e.g. 1.415-5(a)(2)"...applies with respect to limitation years ending with or within the calendar year.