In RRDE we apply two different potentials (one on disk and one on ring) but in one electrochemical cell. So, bipotentiostats are able to apply these two different potentials at the - same time - and measure the current of each disk and ring. The only issue is that these two measurements should be synchronized.

Now, I am wondering whether we can have synchronized timing in multi channel potentiostat or not?

I've got the answer. Yes - we can use multi-channel potentiostat as a bipotentiostat for RRDE experiments. I did not try it yet, I will share my experience thereafter.

Yes, but you can only do this if one (or both) of the two potentiostats is well electronically isolated from ground (floating).  Synchronization between the channels should also be implemented and whether/how to do that will depend on the software/hardware that you are using.