Daniel,
As you can see by the prior response, your initial inquiry doesn't provide enough info to give you a decent answer. Whenever you have a problem with power and a related operating item, it helps to fully describe the power source, how the power is routed to the operating item, and a full description of the operating item. Here's an example:
Power source:
Type of transformer (MTH Z4000, Lionel ZW, etc.) connected to MTH TIU (Rev ?) which connects to layout
Power routing:
List out all your connections and / or all intentional separations (usually insulating pins or other track separations). As a suggestion here, a wiring diagram might be worth a million words.
Operating equipment:
MTH Locomotive (20-xxxx-1) operating under (PS, PS2 or PS3) acquired new (or secondhand).
Finally, it would help if you fully describe what tests you have already performed to try and diagnose the issue. For example, did you try separating the loops to see if the engines would run on only one or the other loop? Did you try just running with only one engine on one loop? Did you try taking any of the locomotives to a local hobby shop to see if they ran there? If you haven't tried any of these, maybe you need to find and read some literature (lots of free stuff to read on the internet) about how your model trains are supposed to work.
Chuck