Hello!

2011/10/13 Antoine Latter <aslatter@gmail.com>
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:

 *** Assembler:
/usr/bin/gcc -I.. -c /tmp/ghc6285_0/ghc6285_0.s -o ../Test.o -fno-stack-protector -DDONT_WANT_WIN32_DLL_SUPPORT
/tmp/ghc6285_0/ghc6285_0.s: Assembler messages:

/tmp/ghc6285_0/ghc6285_0.s:29:0:
     Error: invalid instruction suffix for `push'

Maybe the issue can be resolved by figuring out how to target gcc for 32-bit build.