So basically you confirm what the OP said: Being able to type 2 similar layouts is confusing! This means I made the right choice learning Dvorak years ago. I can switch back and forth easily, probably because they are very dissimilar. I score around 100WPM on dvorak and 70WPM on qwerty, and can therefore use most people's computers without them questioning I'm a computer professional :-)
I think a widely accepted theory over at the Colemak forums is that it's entirely subjective.
As I've switched cold turkey myself I can't offer any experiences. It seems plausible to me that it is confusing using a similar old layout while learning
a new one, however I do think it's easier to relearn QWERTY once Colemak is mastered. Personally I've never felt the need to relearn QWERTY, even though I don't look like a professional when using someone else's computer, as I resort to hunting & pecking mostly.
If I do have to type for more than 2 minutes I install/switch to Colemak and after that I do look like a professional once more.
If I want to look any more pro I let them see my Advantage.
Seriously though, it doesn't happen often enough to make relearning QWERTY worthwhile.
BTW here's a counter example to above theory. Ryan Heise (creator of hi-games.net) maintained his QWERTY skill while learning Colemak, and he also learned at an incredible pace!ryanheise.com/colemak/
I tell myself this is far from typical, because it took me one year to get back to my old speed (to be fair I also switched to the Advantage during that year).