I hadn't seen coinb.in before. It looks like a fun playground, but I would use it for real bitcoin. Uploading a key seems like a bad idea.

You could do the same thing with bitcoin core using the terminal, and you could run it in Signet or Regtest mode so you could try it without using real coins.

The weirdness is that you would have to sign each input separately because they aren't coming from the same "wallet"

I don't know of any wallet software that provides an easy user interface for doing such a thing.