I know a workaround with the exact results you're looking for. Go to the project settings and look for the time base (PPQ). The default should be 96. You'll want to change this to half of the original value, which in this case would be 48. When you look in the piano roll, you'll see that all the notes have been stretched down to 50%, and you'd get the impression they would then play faster. But FL will also play the project at half speed, so it's essentially the same. But if you copy all the patterns over to a new project, they will indeed play at double speed.
Holding shift to manually stretch the notes is also a nice feature, but as said there's the lack of quantization to consider, so much that you'd find yourself having to fine-tune the whole thing note by note.