Yeah, playing both has been no problem for me. In fact, there are still times where I run out of stamina on both accounts. I put more of an emphasis on my US account and when that runs out of stamina, I switch. It works out just fine. You should totally try JP.
On another note, I think I'm going to just level up everything I have currently and save up all my magic stones for the Egyptian gods whenever they come out on US. They make stuff so much more fun.