Explaining MPT is a little hard to simplify and put into words. MPT is short for minimum phase transformed. If you have Cab Lab 3 and you're mixing multiple IR's together it will be easiest with having them all set as MPT. It will make them all work together in a very friendly manner without any phase coherency issues. However, the raw version of an impulse that is not MPT will sound slightly warmer in the mids. Both sound good, and you can decide which one you subjectively like more as the change is fairly subtle, but many people might choose the raw version if they are using the single IR without mixing it together with others for a bit more natural of a sound.
Now, you can absolutely mix together 2 or more impulses that are in their raw form, and in doing so it becomes more like mixing real microphone placements on your sound source. If you've ever done this you might have checked to make sure they are phase aligned, and if they weren't you would match up closely the wave lengths. This can be done in the Axe-Fx, and more easily in Cab-Lab 3, by using the delay knob. Also, if you have the raw IR inside your Axe-Fx and you want to combine it with a different IR that is MPT, all you have to do is set smoothing inside the cab block to 0.01 to automatically make it MPT.
Hopefully this made some sense, and didn't add to any confusion. I can't find any videos unfortunately to demonstrate how aligning IR's works as most seem to show how to mix using the easier MPT method