That said: the 7D is definitely better at ISO1600 than the 5D.
FYI: 5D @ ISO1600
Athlete in Green Room
F/2.8 @ 1/8s @ ISO1600 HH,Spot Meter, Manual, Available (room) Light, AWB.
5D at ISO3200:
"Mother and Child"
F/2 @ 1/25s @ ISO3200, HH, Manual Exposure, Manual WB, Available (Street) Light.
Both images hold very well, to Prints at 14” wide: (open each in a new window and view large)
Regrading having two DSLRs in the kit:
I suggest the leverage for most (non specific non Professional use) would be to have the FL advantage of an APS-C and a 135Format camera in the bag.
Ideally the 7D and the 5DMkII come to mind as the replacement “perfect kit” to the now older 50D and 5D.
There are also other leverages in those two, dual format kits I specifically mentioned; including, but not limited to: functionalities and batteries.
Even if one needs a “back up” camera (e.g. for Pro Weddings Applications), that can be managed having two different formats, by a judicious selection of lenses.
However, I do understand that some would, prefer to use the same camera, and have two different lenses on it: but I think that’s more applicable to Professional shooting under the pressure of time – and even then I think a Pro should be able to get their head around two formats, if the FL advantage suits them so to do.
I think three great advantages of DSLRs are:
> the ability to change ISO
> the ability to make B&W and Colour simultaneaously
> the ability to leverage the telephoto at high lens speed with many fewer lenses, but with two cameras
The last advantage, seems least often employed.