You could do it if the tanks are level with each other, the only thing you will need is a piece of hose to form a syphon between the 2 tanks. Put the intake in one tank, the output in another. fill the length of hose with water, put your thumbs over both ends & put an end in each tank. The water will stay level between the 2 tanks, the tank with output will be flowing into the tank with the intake.
You can only do this if the tanks are level, or nearly so. I would put a piece of nylon screen over the hose ends if you have smaller fish, and make sure the hose is secured good. If air gets in the hose, you will drain one tank while overfilling the other.
If you really want to split the intake & output, the hardware store is probably your best bet for fittings, though you would still want the tanks level with the piece of hose between them in case there is any differences in the split filter flows, which there is no way around that I can think of.
Tolak