Correcting Coverage Failure when Testing Not Otherwise Excludable Separately

We had this question come up several times, but a search of the BL discussion boards did not turn up anything.  Here is the situation.

Different companies in a controlled group sponsor different 401(k) plans.  They intend to satisfy coverage separately.  One of the plans fails coverage even when the not otherwise excludable employees are tested separately.

The question is this:  can they expand the coverage group to bring in otherwise excludible employees of the employer or must they bring in not otherwise excludible employees even if they are from other employers in the group?

For what it's worth, and based on the language of the regs, we are leaning towards the latter approach - that the additional employees must be not otherwise excludible to comply with the description of the two testing groups in the regs.

I would appreciate any thoughts and insights!