This is just physics, you will loose a little pressure but not much due to the length. You will have to at the very least double the output because you are doubling the tanks. The main issue will be that you will require extra pressure on the longer length of tubing.
When I split my air lines (I have like 5 tanks set up with one pump split with 5 T connectors) the longest runs of tube or the ones that are placed deepest or the ones with airstones have the most resistance. I put a valve on every single line so I can dial it in appropriately. If you are just doing one split then a single valve on the line with the least resistance will work just fine.
So, double you CO2 pressure, put a valve on the line that has the LEAST resistance. If the tanks are the same, the depth of the output of the CO2 lines is the same, then the main thing will be the length. Also, the more turns and kinks that go in the line will also cause more resistance.
So once that is all set up it's just down to you CO2 system's output pressure. Then dial in the valves so that the output is even, or whatever you want.