Hello there, actually it will depend pretty much on where you'll be shooting the gig. If you're shooting right up by the stage a mid zoom~(24-70) will do. But for close ups and detail shots, you're probably gonna need your telephoto~(70-200)..
That said, having fast glass is very important so yup, get a 2.8 or faster would help you alot for night gigs. For day-time shows, even a 18-200 would suffice.
Having a fast prime like maybe the 50 1.4/1.8 or even the 85 1.4/1.8 would mean that you might have to move around alot to avoid getting pictures from the same focal length and angles. So yup, my two cents on the lenses.
depends on the images you want, the locations you can access and how crowded the area is as well as how well lighted the place is.
70-200mm f/2.8 works if limited movement but is heavy.
50mm/85mm/135mm is faster but if something happens nearer, your second camera with wide angle better be ready.
be ready to rev up the iso at a moments notice.
if possible, change memory card after each song and ensure the batteries still enough power.
flash is frowned upon but not forbidden unless stated otherwise by management.
security is your best friend.
above are enough steps without going to the more advanced steps which you will naturally find out the more you do gigs.