Its a straightforward question, but there isn't an easy answer.
Lets say for example that we say that only English MPs can vote on English education, welfare, health, or pension bills - all the areas where scotland has devolved powers.
Its very likely at the next election that Labour will get a small majority of UK seats, but not English seats.
So who becomes PM? Milliband - knowing that he can't get pass any education, welfare or health bills in England, because the tories have an english majority?
Or Cameron, knowing that Labour can vote him down in a vote of no confidence any time they want, just by bringing in the scottish and welsh labour MPs?
To make that work, you have to completely alter the nature and institutions of government.