can an account live in 2 domains at once?

Hello,

I was brought in to assist with a domain migration/consolidation project.  The goal is to move users from Domain A and Domain B into Domain C.  However, I was just given a curve ball today.

Client has a LOB app that is tied to their domain B credentials.  the app can not accept domain credentials from 2 separate domains.  so here comes my question.  Can we migrate users from domain A and B into C while leaving a copy of their account in Domain B so that they can continue to access the LOB app?  the # of users in domain B is too large and too geographically diverse to pull off in 1 weekend.

Is this possible?  if so has anyone created a blog or KB regarding how to do it?

Thanks

Sean