Going from 2 things to 1 gives 100% more attention to the remaining single.

The effect will be much higher than that:

Because the brain cannot fully focus when multitasking, people take longer to complete tasks and are predisposed to error. When people attempt to complete many tasks at one time, “or [alternate] rapidly between them, errors go way up and it takes far longer—often double the time or more—to get the jobs done than if they were done sequentially,” states Meyer.[9] This is largely because “the brain is compelled to restart and refocus”.[10] A study by Meyer and David Kieras found that in the interim between each exchange, the brain makes no progress whatsoever. Therefore, multitasking people not only perform each task less suitably, but lose time in the process.

Source.

So, by focusing your attention on a single task instead of trying to do two at the same time you'll be done with that task in less than a quarter of the time (and not half as one would expect).