You need to connect the speakers to your interface and select your audio interface as both input and output in reaper. You should be able to connect them at the back of your interface and each speaker should go into each output (one goes in 1(L) and the other in 2(R). If your speakers both together have only 1 3,5mm cable then you should get a 3,5 to 6,3 adapter (https://images.musicstore.de/images/0960/sennheiser-adapter-3-5mm-6-3mm-jack-for-hd-25_1_REC0008015-000.jpg) and then plug that into the headphone output and you should be able to hear everything