Is it better to combine logic for two flip flops in a single always block?
Using multiple always blocks will make your code modular, and hence easier to maintain and debug, which is essential when the design gets bigger and complex. 

However, it is observed that the increase in number of always blocks slows down the simulation compared to the same logic written in a single always block (See the Source). Otherwise, I dont see any reason why you have to opt out from using multiple always blocks.