If you normally wear your pistol on our strong side (let's use the right side for this example) and you wanted to carry a back-up in a holster in the small of your back you should carry it so you can draw it palm in. Why? Well if it is a back-up think about it, you are under the stress of a situation in which you needed to draw your primary firearm and for whatever reason are now going for your back up gun. Will you remember to turn your hand around, facing the opposite way with palm out, than the way which you just drew your primary weapon palm in, only a short time earlier? Probably not, and that little bit of time for you to realize what you just did wrong, as you fumble to correct, can get you killed. Keep it the same draw for each pistol with the same hand.
There is another reason, and this one applies if you are carrying a solo weapon at the small of your back or if it is a back-up gun. You are suddenly involved in a situation where you (again let's say you are a right handed person) need to draw the weapon that is at the small of your back at 5 o'clock. The thing is, this time, your right arm/hand has been incapacitated, and you cannot draw with it. If you have the pistol in a left handed holster, on the 5 o’clock position of the small of your back, then the butt of the pistol's grip is facing toward your right hip. Try drawing it now with your left hand. Yes you can probably do it but only with some good amount of difficulty. It would be much easier to draw it with your left hand, from that same position, if the grip had been oriented the other way. You could then easily grab it with your left hand palm out (which is the easier movement for your left hand reaching around your back to the other side - try it - your knuckles slide right along your back to the gun). So my suggestion is, right handed shooter, wear it in a right handed holster in that position. Left handed, wear it in a left handed holster but at 7 o’clock.
All the best,
GB