I never go hunting with just one gun. Come to think of it Bow Hunters might do the same, just in case of some sort of break down.
My main reason for two rifles is the area I hunt, it is just plain thick in some places and mixed in other areas. Although most of the shots are close, it is not conducive to Bow Hunting for one reason, you need to put your deer down now because if it runs which it will, it may go into such a thick briar patch you can’t follow. That can happen with a rifle too so you make yourself ready.
For a morning hunt I use a 308 or 30-06 that way if it runs I have all day to track. In the evening there is only one choice, the 45-70 that way with a good shot, it goes down right there. I have had deer run with a 300 and 325 WSM but never with a 45-70 or 50 cal Black Powder rifle.
This is a good move in my book because it makes good sense. Its doubtful if you will lose a deer with your 308 or 30-06 but the addition of the 45-70 gives you the edge. The range isn’t really a factor, generally less than 100 yards, but accuracy is crucial with both rifles if you choose to take my advice. Pass it on.