I'am working on a 2x AD9361 Design (similar to the FMCOMMS5-EZB) and want to keep the design as small as possible. I didn't found any guidelines or recommendation of placment (except the routing guidelines for a single chip).
Do you think that there is a critical distance where both Transceiver can interfere each other (e.g. EMC). On the FMCOMMS5-EZB the distance is about 90-100mm, which seems to be very long. But i don't think that they have design the the eval board to be small (major design-driver will be the FMC connectors).
I kept in mind that there could be some issue with the transmission lines (overtalking) by placing them with a to short distance. From this perspective of view i would say to keep them differential as long as needed and as short as possible.