I need to cross-compile for 64-bit architecture on a 32-bit platform (using gcc-multilib and -m64 flag).
For this I need to install 64-bit versions of the dependent libraries.
How do I do that?
You can't. You will have to manually cross compile the libraries and store them in a non standard location ( so they don't overwrite the 32 bit ones ).
It would probably be easier to just install a 64 bit system and build it natively, or make a debian package and upload it to your PPA on launchpad and let it worry about building it on each of the architectures.
See https://wiki.ubuntu.com/UbuntuDevelopment for more information on packaging and other development issues.