No, you cannot use a splitter at the input to a phono stage, not even the AudioQuest splitter. The reason is that cart has very low internal impedance. When you put two carts together in a splitter, they are effectively shorting each other out.
Your best option other than replacing the phone stage is to get a high quality mechanical A/B switch.
Your best option other than replacing the phone stage is to get a high quality mechanical A/B switch.