You are mixing up some terms here, i'll try to clear some things up.
There is MinGW(32) and MinGW-w64, these are names of projects that supply GNU toolchains for windows. The former is the original project, the latter is a fork or at least based on the former project and considered to be the more current and up-to-date variant. Other than that, these projects are not connected and especially their names are totally unrelated to 32 Bit or 64 Bit builds.
A GNU toolchain CAN be multilib, this means by using the cli arguments -m32 or -m64 it can build 32 Bit or 64 Bit. However, the GNU toolchains of these two projects are NOT multilib. Instead, these projects supply "singlelib" toolchains for each architecture. So to build for a specific architecture you have to use the proper toolchain (what i am not sure is in what architecture these toolchains themself are build, e.g. is the 32 Bit toolchain a 32 Bit application and runs on a 32 Bit OS or is it 64 Bit and requires a 64 Bit OS to run).
This is also true for MSYS2, it bundles multiple toolchains and you have to specify which one to use when you start a shell.