Is it possible for the AD9364/AD9361 to tune each receiver on two different frequencies at the same time?
No, Both the receive path shares the common LO.
Are there any performance or functional differences between the receivers and the observational receivers on the chip? It is not clear to me that there is.
AD936X does not have dedicated observational receivers. Page 56 of the UG-570 doc will give you a rough diagram of the TX monitor configuration though.