My keyboard supports sticky modifiers, I tap them without holding and it modifies the next key. A double tap locks it until I press it a third time.
I use a Keyboard.io Model 100 but I'm sure there are software solutions and/or cheaper keyboards that will do it too. (But shout-out to Keyboard.io, I have all 3 of their keyboards and love every single one like it's a cuold.)
I can type with one hand faster than a lot of people can do with two - ~50WPM (right) and ~45WPM (left) if I don't need to use the modifier keys; "home position" is FGHJ, thumb on spacebar, and from there it's just a matter of memorising where all the other keys are. Quite a useful skill if you need to often operate applications that rely on both mouse and keyboard input.
It was cool because she could use the mouse at the same time, or take notes on a piece of paper.
Since then, Ive practiced one handed typing a bit and Im good enough at it for small tasks. Gets annoying with the modifier keys though.