This is a mixed bag, and probably is, in fact, a multiple-employer plan. Bear with me while I explain. (Thanks for pointing out my faux pas re: deductions for controlled group members, by the way. It helps to read the entire Code Section--actually, it sometimes helps NOT to read the entire Code Section.

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If you look at the proposed 414(m) regs--specifically, 1.414(m)-3©(1)--it says that ASG members are treated as if each maintains the plan when one member sponsors the plan and the plan covers an employee of another member of the ASG, and goes on to say that the plan therefore is treated as a multiple-employer plan. It then concludes that the member maintaining the plan can take a deduction for the contribution made on behalf of the employee of the ASG member not maintaining the plan. Unfortuantely, the proposed regs were issued in 1983, when IRC Section 413©(6) treated all members of a multiple-employer plan as one employer for deduction purposes ("Each applicable limitation provided by section 404(a) shall be determiend as if all participants were employed by a single employer")--but that Code provision was amended into its current form by TAMRA (1988), so the statement in the proposed regs that says the deduction is allowable is probably voided by the change in the Code (which now treats such employers separately).
At the same time, there is no reason to believe that the multiple-employer plan conclusion of the proposed regs is changed. Even though your situation--where 2 members of an ASG participate in the same plan--is not exactly like the wording of the proposed reg., certainly that reg. is consistent with treating your plan also as a multiple-employer plan. Since the regs. would treat it as a multiple-employer plan when only 1 ASG member sponsors the plan but the plan is "considered to be maintained" by another ASG member, why would it NOT also be a multiple-employer plan if 2 members of an ASG participate? So, it would seem, if we rely on the proposed 414(m) regs, that the deduction is not applied on a single-employer basis based on the current state of Code Section 413©(6).
That being said, I would suspect that the multiple-employer plan conclusion in the proposed regs was reached specifically
in order to permit the deduction limits to apply on a single-employer basis to an ASG. The ASG statute obviously did not specifically permit it, but the Code Section 413© rules did. The Service probably saw no difference between an ASG and a controlled group for pension purposes, yet Congress had not applied a combined Section 404 limits to ASGs. So, Treasury got there through the back door of Code Section 413©(6)--a door which closed a few years later with the TAMRA amendment. (I have not looked at the legislative history of the TAMRA change to Code Section 413©(6), but that might be an interesting exercise.) So, you can make an argument, as you suggest, that there really ought to be no difference between controlled groups & ASGs with regard to Section 404 (except, of course, the statutes are different, and Section 404 doesn't apply to ASGs--by statute--as it does to Sections 414(b) and ©). I think that might be a hard argument to make, though (without seeing what the committee reports say), precisely because the statues are different as to Section 404.
Which way to go? It's your plan, so it's your call . . .