There are advantages to having a second 500c. Instead of 500w with the ext, you would get 700w with the two 500c. And, you will always have a backup. worst case, you're back down to 350w. The way to do that is use one amp as the master. That is the amp you will plug your bass into and where you will do the EQ. You run a cable from the Effects Send of the master to the other amp, the slave, into it's Effects Return. Now you have two amps working at 350w each.
Were you to add the two 2x10s to get 1000w, you would hurt people if you're not playing stadiums.
I totally agree with this!!
As you say ( @Blue Dragon ) the 2x combo option works great for a home and rehearsal space rig, and you can always join them together when needed.
But as @Gravedigger Dav points out, 700W of Rumble is a hell of a lot. As much as we can all dream of the "Rumble Wall Of Doom", I can't think any of us could logically name 1 situation in which we could actually use (never mind need) 1000W of Rumble! It is good for fun once in a while to say "Hey,
look listen to what I can do!", but it is not useful in a band situation (unless you are trying to replace Lemmy).
My "big" rig before the Rumble was a 1200W setup, which I could actually increase to 2000W if I wanted, but even when backed up with a 10.000W PA (sometimes even bigger), not once could I turn the rig up above 50% without it completely drowwing out the stage and the first 20 rows of people. I did do it on ocasions during rehearsal (or even a few times at soundcheck) to make the point of "I can make it unbearable if you don't turn down!", but it was disturbing!
I would say that 2x combos, plus maybe 1x extension cab, could be very useful, but more for practicality than volume.
I have yet to find myself in a situation where I need to max the 500C out, and until then, I can't even warrant buying a single extension cab. I would rather spend the money on more bass(es). Don't get me wrong, I love the Rumble and all its tones, but I get all that with just the 500C, what more do I need?
(note: I do have a couple of PA systems, and in 90% of the live situations I run through PA, YMMV obviously)