control group and changing ownership

I will have ot find more information, but I got a request today asking for clarification on if two companies were a control group- there are partners who have common ownership; one partner owns 80% of a related business. They are wondering if he sells his interest, bringing it under 80%, will that be enough? I have a feeling that the other partner(s) have enough ownership to make this idea a non-starter. 

The businesses have seperate DC plans, and have been advised by someone else that they may need to amend those plans (I am assuming this is to provide substantially equal coverage between the two companies).  but I thought there was a grace period for that.

Yes, you need more info, as in who owns how much of each company. 

There is an optional transition period for coverage and nondiscrimination that may be applied after a transaction that creates or eliminates a control group, provided certain conditions are met. The transition period includes the year of transaction and the subsequent plan year.