I think the first thing I would try would be to delete and reinstall the app. Other than that maybe check to make sure the locos each have a unique name. You may also want to check to make sure you have enough voltage/amperage to run two locomotives.
The App controls Bluetooth equipped locomotives. It does not control DCC locos. DCC and EZ App locos can run on the same track at the same time. You just cannot control an EZ App loco with a DCC controller and vice versa.
The Power Booster is primarily to boost your track power amperage. As stated previously your best bet is to use any standard HO power pack to power your accessories. A small HO pack with an accessory hookup would be ideal.