Yes they can, they both could use the same System memory. Also, If the ARM core can use HuMA it could work with any other arch.
No they can't. AMRv8 and AMD64 are different scalar ISAs with different TLBs. For hUMA you must use the same TLB for the LoC and the ToC.
Also there is no reason to use two different scalar ISAs in one chip. If AMD will use an ARM core in their nextgen products than it will do some special stuff, but nothing more.