When I am changing channels on my Premiere and Roamio basic, it continues to use the same tuner. When I am surfing like that, I am changing to programs in which I am interested. I am not necessarily ready to commit to recording them when surfing but if after turning away I choose to go back to a prior program, I miss everything that happened previously (unless I had chosen to record it).
I am aware that hitting the Live TV button switches tuners but I think that the Tivo should automatically switch to an unused tunerwhen I switch channels so that the channel from which I am turning continues to record the 30 minute buffer. That way, if I switch back 15 minutes later, I can rewind and catch what I missed.