First and most important, you should not use the 50T chainring with the 27 or even 24T cogs. It's called cross chaning and is hard on the chain and there is no benefit to these gears. A 34x19 gives you almost the same gear as the 50x27 and 34x17 is a good substitute for the 50x24. Second, a double shifter and fd is not going to solve this problem.