I have a remote rad set up , not to your scale , 8'down -8'up
-I put 1 655 in the case and 1 at the rads to balance the push\pull pressure most pumps push better than draw.

-can't tell by the pic's if both pumps are at the top[ up stairs] or low ,[downstairs]
-Try moving one pump downstairs and one upstairs -I take from the pic's your upstairs and the tube crush[crushed by air pressure-not sucking] is from pulling the water up to that height maybe.
just saying
-maybe cpu[system]>inlet upstairs pump[1\2"]outlet>[1"]down to rads>[res.?]inlet lower pump >[1\2"]up to system[1"] ?-might make up for the thin wall tube-open for debate for sure.