Re: have you ever gotten the SAME amp as you already own as a backup amp?
I really believe that any guitarist who is "locked in" on a certain model of guitar to get his tone shold have a Number ONE-A--a guitar that he can pickup and play and be totally happy and comfortable with when he doesn't have his Number ONE. My Number ONE is a G&L Legacy, and I have several Legacys to choose from. I don't worry about the maple or rosewood neck difference--I'm fine with either; but it has to be a Legacy. My second guitar is a 335 or a Les Paul (depending on the gig), and I have backups for those, too.
So, I think the the concept of a Number ONE-A amp is also relevant for certain players. I have one friend that tours, he's a diehard Marshall man--and he has a second JCM 800 that sounds "close enough" to his Number ONE. Of course, he has some other amps too, but he's like me with my Legacy--I don't think he could do a gig with a Fender amp without a nervouse breakdown, LOL.
I have five Mesa amps: DC-3 combo, Maverick 212 combo, Mark IV combo--and three Mark IIIs. Two combos and a head/412 half-stack--they do sound a little different, yet they are "familiar". With the gigs I'm doing with my current band, I have a nice selection of amps that allow me to cover a wide range of styles and venues big and small. And while I may have a favorite amp, I'm happy and comfortable playing on any of these amps.
And I doubt I'd be happy trying to play a 4-hour gig with just a JCM 800 Marshall or Fender Twin. I'd have to make some big changes to my pedalboard, to get by with a single channel amp. Indeed, we all know that some pedals work great with some amps, and not very well with others. That alone would give me hesitation for using a different kind of amp.
So my recommendation would be based on how picky your are. If you don't mind playing on different amps, fine--build a stable of several different amps. But if you are locked into that Marshall, Mesa or Fender tone and can't bear the thought of playing anything else--you'd better have at least one backup for your main amp.
Bill