1. If the clocks are of same frequncy but from different sources, then you need to check if there is any ppm diff. between the two clocks. If there is any diff. then even using an asynchronous fifo won't help. As ther fifo may go empty/full after some time. You need to design special FIFO for it.
3. If there is no ppm difference, then using an asynchronous FIFO is a good option.
4. Usually flop based sync. techniques are not used for data bus as it every bit of data may be in diff. phase.
Regards,
Sanjay