Cross-posting here because I missed this thread when I started mine.
Long story short, I’m trying to do exactly this: Repackage a deb app as a snap. So I need to lconsume either a x86 or arm64 deb as source, depending on the target. The suggested snippet, which is also what I tried, seems to indicate snapcraft to use different sources when building on different architectures instead of for different architecturs. So it works as long as you build you x86 snap on an x86 host and the arm64 snap under a arm64 host.