note that multirec depends on the programs being on the same physical channel. ie one is on 4.1 and one is on 4.2.
I recorded 4 different HD shows from four different networks (5.1, 11.1, 17.1, 50.1), and they all recorded fine. So not sure if what you stated is accurate.
Multirec works as follows;
A bundle of Digital TV channels are broadcast on a given frequency - ie they are multiplexed together into a bit stream on that frequency. When you request Channel 10 Myth chooses an available Tuner and tunes to the frequency/multiplex that Channel 10 is broadcast on and it extracts the channel 10 stream from the multiplex on pne of your 'free' multirec virtual tuners. Now if you choose to watch or record a second TV channel on say Channel 30 the same process happens...except this time Channel 30 is on another frequency/multiplex. So now we have both physical tuners deployed delivering channels from two separate multiplex's. In this situation while we are watching Channel 10 & 30 we can also record up to 4 other channels on either multiplex...but we cant watch/record any channels on other multiplexes until we free up one of our physical tuners so that it can be tuned to that frequency/multiplex.
Hope the above is useful.