I've done it without any problem.
I used to have a D* R10 that would frequently be recording two shows while we were watching a prerecorded third show. The drive was dealing with data for three shows simultaneously.
With a standalone, it can also deal with three shows at a time max. Watch recorded show, while new one is recording, while transferring to/from computer. It may even be able to deal with more at a time depending on how many computers or MRV tivos are trying to access it, but in those cases, each machine would only get a fraction of the available network bandwidth thereby reducing the strain on the tivo drive by the same amount. In other words, no matter how many shows are being transferred across the network, the tivo can only go as fast as its hardware limitations or the network limitations.
Jeff