Have the main tank drain into the 20 and then have a pump in the 20 to supply the 10. The 10 then needs to gravity feed back to the 20. Don't put the main pump return in the 10. Won't work.
Otherwise it will be fine. I ran a similar setup for my 75. 75 to a 25 gal
sump. Maxijet 1200 pumped from the sump to a 25 gal
fuge. The fuge was raised and it drained back into the sump. I could turn the main pump off an still leave the sump/fuge pump running.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,