It looks like some applications will be flagged as multiarch, so maybe
you'll be able to install two GHCs side-by-side with apt.
I used x64 edition of Ubuntu 11.10 for a while, and was able to run both 64-bit and 32-bit GHCs. Both GHCs were manually set-up (i.e. download; tar xvjf ...; ./configure --prefix=...; make install)
All is needed is to install both versions of libgmp by "apt-get install libgmp3-dev lib32gmp-dev"
32-bit version runs ghci fine, but I actually cannot compile stuff with it. Haven't looked deeply into that issue, though.
The error is like the following: