Again, it depends on the Software. Reaper (on my PC) for example shows all MIDI devices connected to my Computer or allows me to add a Joystick as MIDI device for instance and also allows me to add multiple "control surfaces", so I've got my FaderPort & Nektar Panorama P1, etc... those aren't MIDI per se, but can be used to control MIDI if need be... but that's all handled in the Software. I believe on my iPad (assuming Android would be similar) I have to actually choose a MIDI Device (in my Synth App). However if the OS (Google?) can see all the Devices itself (I don't know how Android/Google handle this stuff, but it wasn't made with Audio/MIDI in mind...) then it's possible you just need to select the MIDI Channel that you want to "listen" for in your Software (App) of choice... perhaps you can find a DAW for Android (N-Track? Cubasis?) that will be able to see both MIDI devices, but I don't think it's like iOS where you can insert a separate Synth App for instance into your DAW... so you're still stuck with whatever your Synth App can see and use for MIDI, which you might need to choose one or the other in the settings of that particular App.
Edit: Which App are you trying to use? Perhaps it's possible to reach out to the Developers and explain what you're trying to do. Either they can help you configure it to work, or perhaps they can Update the App to allow multiple MIDI devices to be used at once.