Mr Brown has it right, I do it often during the night when I don't have time to fill the right reservoir. You have a normal and a alternate antiskid/brake valve in the wheel well. The alternate uses left hydraulics, the normal and reserve brakes use right hydraulics. Left and right are a separated system, no fluid transfer, BUT, the two antiskid/brake valves connect to a brake metering valve, that chooses which brake valve to operate (normal or alternate). So between the brake metering valve and the brake itself is only 1 connection, which can contain left or right hydraulics according to which brake valve working. It is this fluid in the brake that you can exchange between the 2 systems. Since center hydraulics are not used in braking, you canot exchange fluid from the center system.
And for the fluid loss, the max drop I've seen during fluid transfer is 0.9, so not a big deal if you service the reservoir correctly first.