If 2 PLLs as the one shown are driven from the same reference/crystal and the ratio between divider N and R is an integer (note that divider R is outside the loop), does one need a sync pulse for divider R or not and why? what if the division ratio N/R is not an integer?
Yes, in order to create multiple PLLs that are phase-coherent with one another, then you need to specifically design in some functionality to implement that synchronization.
The issue is that each of the dividers in the block diagram that you showed can have multiple stable operating phase offsets (phase offsets where the input/output frequency relationship is met). If you don't specifically design the system such that those operating points are deterministically chosen in some way, then there will be some residual phase offset in the output based on the randomly-occurring phase offsets in the divider outputs.