spec file question; build dependency on g++ -m32 in x86_64 arch package

I’m having difficulties specifying a “BuildRequires” type dependency in my spec file.

I am building a x86_64 package, but would like to also build some of the files with g++ -m32.
I believe the packages I depend upon are “glibc-devel.i686” and “libstdc++.i686” and these
are not normally installed in Fedora 64-bit.

I tried specifying this dependency in the spec file as:

BuildRequires: glibc-devel.i686 libstdc++.i686

and also as:

BuildRequires: glibc-devel(x86-32) libstdc++(x86-32)

but neither are found.

Any idea what I’m doing wrong or if there is a different method to accomplish this operation?