I have a cash balance plan with 4 HCEs and 10 NHCEs.

2 HCEs want to leave the company and start their own company.

The plan has a shortfall.

Can the 2 hces that are leaving waive part of their benefits so that the other 2 hces aren't on the hook for it?

I looked at a similar situation and concluded that the only option for a benefit waiver was a plan termination. If they are just terminating their employment, and the plan is continuing, I don't think they can waive it. However, if it is underfunded you can't pay them a lump sum anyway.

What we looked at was an arrangement where the doctor who was leaving agreed to pay the company some dollar amount. If they are owners and are selling their shares back, you may be able to adjust the share price to account for "their share" of the shortfall.