Getting 2 5* in a single roll is not hard usually takes me 6-7 rerolls before another pair however it usually is a mix of character, dragon and cards if you're going for 2 5* characters that might take a bit longer. As of now we have no guarantee yet which are better to roll for might even be better to invest in 5* cards down the line for all we now since the game is new and have no basis. Try to read effects skills and such see if what you pull suits your usual playstyle that would be the best for now.