You don't need to create separated packages name to get multiarch. Just type:

Package: foo-amd64
Version: 1.0-1
Section: base
Priority: optional
Maintainer: [email protected]
Architecture: any
Multi-Arch: same
Depends: foo, libgcc1, gcc-4.6-base, libstdc++5, libstdc++6, libsasl2-2, libgstreamermm-0.10-2, libfreetype6,
 libfontconfig1, libX11-6, libXrender1, libice6, libxext6, libsm6
Description: foo is awesome
 Foo is really awesome
Installed-Size: 71140

That should build both, amd64 and i386 packages depending your build environment variable. There are various ways you can convert your packages to multiarch if you use the recipes instead:

dh(1) and autotools

The simplest package to convert is one that uses autoconf upstream and dh(1) in the Debian packaging.

1. Build-depend on debhelper (>= 9).
2. Add Pre-Depends: ${misc:Pre-Depends} to any package listed in debian/control that provides a shared library.
3. Update debian/compat to '9'.
4. Replace occurrences of /usr/lib/ in debian/*.install with /usr/lib/*/.
5. If /usr/lib (or a subdirectory) is listed as an installation target in debian/*.install, or the target of a link in debian/*.links, you will need to substitute the value of $(DEB_HOST_MULTIARCH) into this file. See `Dynamic debian/* files below.
6. Any occurrences of /usr/lib in debian/rules should be replaced with /usr/lib/$(DEB_HOST_MULTIARCH).
7. If you needed to use $(DEB_HOST_MULTIARCH) in debian/rules in either of the two preceding steps, set DEB_HOST_MULTIARCH in debian/rules by calling: DEB_HOST_MULTIARCH ?= $(shell dpkg-architecture -qDEB_HOST_MULTIARCH).
8. Once you have built the package and verified that the shared library package contains only the expected files, and that your -dev package still works, mark this shared library package Multi-Arch: same in debian/control.
9. Mark the common package(which is "all") Multi-Arch: foreign in debian/control, if it may be needed by the shared library package.

If you have overridden debhelper commands in debian/rules, despite using dh(1) for the rest of the build, you may need to follow the instructions below for classic debhelper to modify mv, cp or rm commands used by your override rules.

There are more examples in the Debian packaging implementation guide https://wiki.debian.org/Multiarch/Implementation. You may also use lithian to make sure your package in completely compliance.