Little inch and Novice hit it. Think of it this way, and I'm thinking of a closed loop system (not domestic). The fill pressure is the amount of pressure to lift the water to the highest heating/cooling element in your building with no pumps operating. I then add what I call a flood pressure, making sure I don't suck in air anywhere. I typically use 5.0 PSIG, this number can vary depending on the engineer. (nobody tell me this is wrong because I have 25 years of 'cplaining to my clients if I'm wrong). For example; You have a 40' elevation to terminal unit, then I would charge the tank at 17 PSIG (40/2.31) + 5.0 PSIG or 22.0 PSIG. This assumes a specific gravity of 1.0 which even glycol systems are pretty close to that.
You did mention one is on the supply line and one is on the return line, and I'm hoping on different systems? You only want one expansion tank in your system, think of it as an electrical ground. Normally the expansion tank is located on the suction side of pump(s). I'm not saying what you have is wrong, just curious and believe me I've designed stuff that had tanks tied to EVERYTHING. (Won a bet on that one). People get crazy on that it has to be right next to the pump, it really only needs to be hydraulically close to the suction side of the pump. Not right next to it. A pump with an expansion tank on its suction side will add its pressure to the system pressure. An expansion tank on the discharge side of a pump will subtract it. If placed on the suction side the system will be operating w/ a positive pressure, even at the highest point. If the tank was on the discharge side, it would draw down the pressure to obtain its pressure differential, possibly sucking in air at the high points.
Hope this is helpful.