There's not a simple way that I'm aware of. If you're an owner/moderator with access to the Member list, you can go to the member name, then Activity History, and search for Accepted Message. A kluge at best, but works for now.
If you and/or the other person always sign the same way, you could search for that in Message View, but it would also show any quotes that contained it. (Search won't find them in Thread View unless it's in the title.) I think Mark has plans to add a search by poster.
Duane
Duane