I have two Nikon FE2, plus an FA; so two identical bodies plus a very similar one. Then I use a D300 and now a D700. There again, two very similar cameras. I haven't had the D700 very long. I think I would either carry the D700 and keep the D300 in reserve as a backup; or possibly keep a different lens on each. But with the Nikon DSLRs, they are heavy, especially the D700. So I think I will, on road trips, use them mainly for shooting near the car, and not for long hikes!
Leicas: two M6TTL with .58 finders; an MP with .58 finder; M6 classic and MP with .72 finders; an M2; and an M3. And now an M8.2. No, I don't bring them all!😱 The most I have ever brought is three M bodies. Oh and I have three IIIc.
My main reasons for carrying more than one film body is either to have different films loaded; or else (in the case of the M bodies) to have two different finder magnifications available.