None of the DVD recorders I've seen (DVD/VHS or DVD/HDD) allow simultaneous recording on both sides at once: you choose one or the other. I don't think there's any technical reason why they couldn't, the feature must simply not be thought of by mfrs as something consumers demand. OTOH, all the recorders I've seen do allow you to play anything you want on one side while recording on the other, which is a very useful feature for many people.
The only recorder I've ever seen that could record on two media simultaneously was the first JVC DVD/VHS combo, model DR-MV1. This unit went as far as having separate tuners for the DVD and VHS sections, so you could record different channels at the same time. I was always rather surprised that didn't become a popular feature, but it was gone by 2005.