What type of problem occurs when paralleing two MOSFETS with the same frequency and Phase?
Does placing inductor in source of both transistors is also a remedy? If yes then why?


A remedy for what? What problem are you trying to solve with that? If you don't know what the possible problems are, then how did you come up with this solution?



Potential problems of parallel transistors:


Oscillations and ringing between the gates of parallel transistors (for transistors that have gates)
Different rise/fall times causing some transistors to conduct more than their fair share of the load during switching.
Thermal runaway due to non-identical transistors and positive temperature coefficients causing some transistors to steal the current from all the other transistors and conduct all the current themselves. Some transistor types are better at this and some are worse than others.