I've tried to see the case for having split-screen apps and I just can't see it. On x86 you can use the normal Windows 7+ screen snapping as well as resize the windows to your choices. On ARM you .... can't run desktop apps so you're stuck with apps and this is the only way to run two apps....... ah now I see the point! Try to make the ARM experience a little closer to the x86 one and hope that users will accept it.
Aside from price concerns this is the single biggest reason to go for x86 over ARM. It seems not even Microsoft have a clue what ARM / RT is best at since their ads focused on some shity High School Musical inspired ad with lots of dancing and showcasing a stand that clicks.