I routinely have a BUG but it is the same caliber as my EDC gun. Specifically, I carry a Kimber 1911 .45 ACP Pro Carry Stainless. My backup is a Kimber Ultra Carry 1911 .45 ACP. I also carry two spare mags so the mags are good for either gun.
The only change to my routine is if I am going to be in an area that is environmentally unfriendly to metal guns and 1911's, i.e. extremely wet and rainy or lots of blowing sand. For these situations I carry a H&K .45 USP Compact and my BUG is a G36. Again, both are the same caliber. Unfortunately they do not use a common magazine but at least I can rob cartridges from one magazine to feed the other. Granted, it would be better to have two guns that can feed from the same magazine but I can say, with confidence, that my H&K has never had a failure of any type so I do not see it as anything but a minor inconvenience.
I train with these four guns all the time and carry each in its respective holster in its respective place all the time.
May the Schwartz Be With You.
NRA Life Member