Use mstsc /l to see what monitors Remote Desktop sees. Note the monitor numbers that you'd like to use for your remote session.
Find your RDP config file (usually C:\Users\<username>\Documents\Default.rdp) and add these lines (if they don't exist). The main one is selected monitors. Substitute 0,2,3 for the monitors you'd like to use based on the output of mstsc /l.
span monitors:i:1
use multimon:i:1
selectedmonitors:s:0,2,3