You just can't play audio from both apps at the same time but you can certainly alternate between which app is active.

I seen someone recommend getting the Behringer driver from their site and using that to fix latency, so I did. However, I noticed that there was significant latency. You can also select in the preferences inside of Reaper, 'Close Audio Device' when Reaper is not the active application, thus when you select another app to play from, then Reaper will close the ports of the ASIO driver for the sound card as long as Reaper is not the active application. After several emails back and forth, Behringers support finally admitted that the UMC404HD is not tuned for ASIO and the asio4all driver doesnt work is not. I just got this Behringer UMC404HD interface and it was plug and play with the automatically found driver in Windows 10. Chose which works best for you in the Reaper Sound Card settings. ASIO is more of a professionally used driver, but it is limited to 1 app at a time accessing the sound card. Microsoft Sound Mapper is commonly selected by default in other apps because it is most compatible when audio is played from multiple apps, but also provides the worst audio latency problems with the specific app that is using it.

Something like WDM, WAVE and Microsoft Sound Mapper drivers don't have this same problem of sharing audio between apps and is likely what the other apps have selected by default. If you want a quick fix, then go into the audio device options inside of Reaper and select a different sound card driver for the UMC-404HD. Which driver are these 'other apps' which don't have this problem using? After I downloaded (search for ASIOAll) and installed it, in Audacity (Edit->Preferences>Devices) select Windows WASAPI for Host. This is a very common problem with using ASIO drivers, which Reaper will select by default since it provides the best latency performance.

Behringer umc404hd asio