Can a company merge an underfunded DB plan with a related (same sponsor) overfunded plan to eliminate the underfunding? The underfunded plan has already been granted a waiver of the minimum funding standard.

Might have a problem if either one is union-negotiated, but this is probably a viable technique. If you have an outstanding waiver of minimum funding standards, you might need to run this by the IRS.

But remember 411(d)(6). Make sure the merger does not "injure" the participants in the overfunded plan.

I don't think it is a 411(d)(6) issue. I think it is a 414(l) issue.