Well, they don't "work together" - Wine devs avoid ReactOS code and devs like the plague. However, Wine is happy for ReactOS to use their DLLs - so Wine does Win32 and ReactOS does the lower, weirder layers that Wine doesn't bother with.
Is this due to any patent/copyright issues?
Also, Wine is LGPL and ReactOS is GPL, so no code can flow in that direction.