Most effective way to level with same character but different star level?

I have 3* Masha and Canna ready to rank up (just need rank up stones) and 4* Masha and Canna level 1. How should I exp synth them? Should I wait until I get enough rank-up stones (speaking of, can I only get them from daily dungeons?) or just exp synth them to the one with higher stars. Thank You!!!!