Like Wilson said ~ just stagger the tape start times. Get one rolling a few minutes before you think the show's going to start (it'll probably roll for at least five minutes before the show actually does start), and get the second one rolling right as the show starts.
The 80 minute tapes are really nice (and run for 120 minutes in LP mode), but you pay a lot more for the extra 40 minutes as the tapes themselves are considerably more expensive than the 60 minute tapes. Of course, if you really need the additional tape and it saves you a tape switch, it might be worth the extra expense. However, a lot of times, you end up not needing it - so the additional money spent is pretty much wasted. If you have a really good idea how long the band is going to play, then you can make an informed decision - otherwise - I'd stick with 60 minute tapes and save money.