How are the RPis connected network wise?
When referring to versions I mean the ServerWMC backend and pvr.wmc frontend versions, rather than your XBMC/openelec version. There is a newer backend version available, and also the frontend addon has a newer version than that which is included in Gotham Beta because we had feature enhancements which couldnt be included in Gotham due to feature lockdown prior to going into Beta. So you need to manually update the pvr.wmc addon on your RPi's if you want that latest version.
In terms of your freezing, I dont think it's "the norm" but despite your server PC definitely being a few generations old now, I wouldnt be comfortable in saying a newer machine would definitely get rid of the issue. My setup is a "mere" Sandybridge Pentium G620 with USB TV tuners and a iSCSI mounted drive from a NAS used for TempSWMC and recordings, all over gigabit LAN. To my knowledge I dont have freezes when a 2nd client starts watching a channel although I cant say we do alot of multi client viewing. Still, one would assume that a sandy bridge pentium wouldnt be smashing a core2duo, and similarly a SATA attached 1TB drive should be OK if a GBLAN connected iSCSI NAS drive is... You certainly dont NEED an SSD for the TV recordings, although obviously that would support more concurrency and higher write rate but it would be too expensive to use SSD for the large storage requirements of raw recorded TV. Perhaps a power user setup is an SSD as scratch/recording drive, then on recording completion automatically move the recordings off to spinning platter storage or a NAS etc. I dont have any experience with network tuners, nor this MoCA connection. If you want to get to the bottom of things, I think you would need to do some mucking around. I would suggest starting with the the server PC, a network tuner, and 2 rPI's all plugged into the same GB switch, and see if you can replicate the issue. Also if you have any other alternative serverPC (even a laptop) with better specs, you could always give that a go as well.
Also as I said, post up a ServerWMC log, there could perhaps be some error message or something logged that points the finger ina particular direction.
We have also seen some pauses/slowdowns when a GetRecordings request came in from a client and the metadata scraping seemed to load up the server affecting it's current streaming jobs. This is dependent on how many recorded items you have etc though, but also this feature was disabled in the latest build 1134 (perhaps another reason you should update your backend)