I'm a touch typist. Both thumbs are stationed over the spacebar, yet I primarily use my right thumb. That may be a factor of the frequency of keys I happen to use on qwerty keyboards, or may just be predilection.
My favorite keyboard for typing was the keyboard used on both the IBM 2741 terminal and the IBM Selectric typewriter. I loved the thing. Because you could "buffer" key presses -- if a key was already depressed, a second (and third, fourth...) key would lock awaiting release of the active key to engage. I could type faster on one of those than anything I'd used before ... or since.
Oh, yeah... I'm right handed. But touch typing has been engrained for .... 44 or 45 years, now.