That is strange that you and your partner didn't get put into the same lobby, but it may depend on how closely in sync you were with starting your matchmaking. Once a lobby is created for you (usually ~30 seconds after you begin searching if it can't find a suitable lobby to bring you into), it won't "merge" with other lobbies that are already created -- this is why when your partner had their own lobby created, it didn't bring you two together. If you want to play together, it's best to join a party through Steam and then start matchmaking.
As a reminder, we do have a Deformers Discord server where there's a mixture of PC, XB1, and PS4 players who tend to regularly party up and play together. It'd be great to have you join so you can find others to meet and play with!