We have both --targets (plural comma separated) and --to-feature (singular allowing multiple options) in the interface here. Either is fine, but might make sense to have them be consistent (either plural or singular but not both)? So eg.
swift package migrate --target Foo --target Bar --to-feature Baz --to-feature Boop
or
swift package migrate --targets Foo,Bar --to-features Baz,Boop
Could check what we do for other commands, though I suspect we may be a little inconsistent there too 