Here's how you activate Ch 3/4 with URRT4 in Reaper

Reaper - this is how you activate Ch 3/4 with URRT4 -
Added Main Outs - MIX 1 = Ch 1/2
Added Main Outs - MIX 2 = Ch 3/4

URRT4 - Sound is Powerful + Golden!!!
URRT4 - MAIN OUTS - Power Hafler amp NS10 Speakers
URRT4 - 1/2 Line Out - goes to a Monitor 2 USB to MONO Avantone Speaker
URRT4 - 3/4 - Line Out - goes to a Rupert Neve Headphone Amp to Sony MDR-7506 Headphones