250W seems underpowered for a 2x12". Nothing wrong technically as such, but it seems inefficient to be lugging around a large cab that you can't then make the most of. As for mixing cabs, it is indeterminate whether the MB115 will sound good in combination with your choice of 2x12".
Instead of ending up with three pieces of gear, potentially none of which will go that well together, how about selling the MB115 and LM250 and picking up a more-powerful head which is better matched to the 2x12"?
Or - buy a second MB115 (or 115MBP if you can find one - the discontinued powered extension). Having two combos also provides the backup you mentioned.