After some testing last night, bi-directional MRV is still a no-go. Audio cuts in and out, video drops, etc when watching a kids animation from one receiver to another while watching monday night football in the other direction.
In looking into the tivo HD forums, where the tivo HD uses a similar cpu, chipset and network interface as the HR2x's, tivo users are seeing that the platform tops out at between 15 and 24Mb/s of total throughput. So its possible that each HR2x will only be able to handle one MRV/directv2pc stream in or out at a time, which probably explains why they limited the streaming to one stream. I'm guessing they might end up limiting to either one in or one out, but not both because it just doesnt work unless both streams are SD.
In my case, its two HR20-100's (which have somewhat faster parts vs the HR21/22/23) hardwired to a gigabit switch thats cascaded off my router. Just cant get any better/faster on the physical end of this.
Whats reallllly strange is in looking at the tivo folks results, there is also a pretty wide variance in what people reported for max throughput with the same tivo hardware...15-17 for some, 20 for others, 22-24 for others. My guess is that each of those had some very different things going on in the box, like more season passes, more wishlists, more guide data, more channels, etc.
Which means when the HR2x is doing more stuff than just sitting there spooning out an MRV stream, its total possible network throughput may drop. If it drops below ~15-17Mb/s, even a single HD MRV stream might start to glitch.
So I think I might have figured out why some people have glitchless MRV and some have it glitch so bad its unwatchable. Once physical network issues are resolved and removed from the equation, it may just be that some peoples demands on the box are higher than other users, and their MRV glitchiness is correlated to that extra work.
A user with 50 series links on both boxes, recording both tuners all the time (or even an extra OTA on the 20's), doing VOD, lots of guide data processing, searching for stuff, keyword autorecords, etc...vs someone with 10-15 series links that record once each a week and dont do much else.
My experience with disconnecting my receivers from the internet and just cabling one directly to the other improved MRV glitching and overall performance, so that fits in with the limited throughput/limited load theory.