You can resize , 50-50 on the 12.9 I believe is = 2 iPad mini's
I don't have the 12.9 but I'm sure I've read that that is one of the advantages of having the 12.9 over the 9.7 or 10.5 as you get 2x iPad layout instead of 1x iPad + 1x iPhone
Someone who has tried the 12.9 can verify and correct if needed