Welcome to the forum!
You may be able to run both sets on the same layout using only one power supply, although you may be pushing the power limits of the power supply doing so. You can try and see the results. You do not need to have 2 power supplies for the 2 trains on the same loop, and hooking both power supplies up to the same track gets involved.

It would be advantageous to upgrade to a bigger transformer. Postwar transformers such as the KW can run 2 trains, and the ZW can run up to 4 trains; both with power to spare. These transformers are widely available at hobby shops, train shows and online auctions for very reasonable amounts.

Note that if you get another LionChief set, the locomotives must be a different type, or you will not be able to control both locomotives independently.

Larry