This is why I have a narrow keyboard, and therefore the any two of Keyboard, mouse and tablet can be easily used at same time. I often tend to type with pen in my fingers if I'm using tablet, as I am now.
The new Mac keyboard is nice and narrow, albeit cramped, but I use a Logitech keyboard that has a seaprate number pad as it's narrow, but also very comfortable to use. And less fiddly than Apple's keyboard.
Digressing, but also ergonomically related.
I've never understood why Apple insist on using cramped Laptop size keyboards for desktops, though I suspect they save money by having less sometimes variation in kit even though it's at customers' expense, comfort wise.
Also using the same keypad on 12" laptop as the teatray sized 17"! Cheap or what?