From the documentation about
Specifies any parts that should be built before this part, which Snapcraft then stages before trying to build this part. This is useful when a part needs a library or build tool built by another part. If the part defined in after is not defined locally, the part will be searched for in the parts repository.
That means your other snap does not qualify, because 1) it’s a snap, not a part; and 2) it’s not included in the remote parts repository.
To use another snap at runtime you need to use interfaces - plugs and sockets - to link the other snap into yours when your snap gets installed.
To use a snap at build-time there has been mention of a then-undefined
build-snaps stanza, which was talked about as potentially working like
build-packages. I don’t know whether that was settled-upon, included in any releases, or abandoned as a silly idea, so I might be sending you on a wild goose hunt by mentioning it