Assuming the tall tank fills first and then the shorter on fills from it. Seems to me that both tanks should flow equally when water is drawn by the pressure pump. Ie. They both should empty at the same rate.
Fill both tanks and isolate the taller tank. See if the pressure pump draws water. Should be able to eliminate the taller tank's out flow line to the common line and, draw only from the lower tank with the taller tank flowing into the lower one as needed.
'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton
Please edit your profile with a name and location so we can better assist you and make for a friendlier forum.