To continue to use your two Arcam AV888 receivers as 5.1 channel surround setups in your two rooms, as well as be able use them for distributing audio with audio|acacia, I would recommend the following:
The Arcam AV888 receivers appear to have 8 channels of discrete analog inputs (labeled as "MCH INPUT" on the back of the units). Since you are only using the receivers for 5.1 channel surround, this means you have 2 extra amplified channels per receiver to use for multi-room audio speakers. I haven't looked at the receiver's manual, but assuming you can set, say the SBR& SBL discrete inputs to a different zone (ZN2 or ZN3), you could actually setup dedicated multi-room audio speakers that will work even when the receiver is playing 5.1 audio from a different source.
The speakers that are already setup for the 5.1 surround rooms themselves could use L&R inputs from "MCH INPUT" to route to the Left & Right speakers in the rooms. Then to switch between multi-room audio and 5.1/7.1 surround for your rooms, you'd simply switch the receiver's input from "discrete", to whatever source you wanted audio from (i.e. SAT/PVR/DVD etc.).
Multiply by two receivers and this gives 4 possible rooms. 2 Rooms dedicated for multi-zone audio and 2 rooms that can switch back and forth. (This assumes you'll be able to route cables to both receivers from one audio server).
Since you'd be using the discrete audio inputs (i.e. no digital processor), there should be absolutely no perceived latency involved that would cause sync issues.
In the end, you now have 4 rooms of multi-zone audio without having to pay for any extra hardware (assuming you already have speakers). Not too shabby. ;)