I agree with codifies when it's about "muscle memory": ten-finger muscle memory simply does not translate to two-thumb muscles. If it would, then you could only type spacesMy thumb typing on tablets doesn't have any "muscle memory" from a full sized keyboard, just because a full sized keyboard does it that way is no justification for doing it on a smaller keyboard, potentially quite the reverse!They all come from the right side of the US standard keyboard. If you use one, that will be where you instinctively look there to find them.
I agree with Grench when it's about "knowing where to look".
Whatever we do, muscle memory is not going to be very useful for our purposes, but "mind memory" (which is just regular memory I guess ) can still be exploited. That's also the reason why nearly all of us want to have QWERTY and standard shift+number symbols.