You can split the line or switch back and forth. When I was in this situation, I would charge the newer keg with higher psi so the CO2 would have some time to dissolve before I needed to hit it again. However, I got a 3-way manifold very soon after, as I was expanding my setup rapidly. Now I also have a dual-pressure CO2 regulator and a separate line coming in for that, so I can serve at one pressure and carb at another. Good luck.