I avoid QWERTY at all costs, so my QWERTY speed might be around... I've not actually measured it since the switch. Well, it seems to be about 25 WPM. When I started out with Colemak a little less than two months ago it was at 70-80 WPM. I have no intention to maintain proficiency in both layouts, so I might be the wrong person to comment on this matter, but it should be possible to type using both layouts. If you with to do this by first learning Colemak and then introducing QWERTY again slowly, or integrate QWERTY in your Colemak learning, I guess is up to you.

Also, a link you might find interesting: http://www.ryanheise.com/colemak/

I managed to maintain proficiency in both layouts by alternating between them from day one using a "qwerty by day, Colemak by night" approach. Once I was fast enough to be able to use Colemak at work (over 30 wpm) I switched to using it pretty much full time, except when I had to log into servers etc.

At first it typically took me ten minutes or so to switch between layouts, but these days I find it's pretty much effortless. Having said that, I do find myself typing Colemakisms when I have to use a colleague's computer to show them what to do, though that doesn't really bother me.

Since you're up to about 20-30 words per minute you're probably pretty close to the point where Colemak "clicks," and you can touch type in it without having to stop and think where such and such a letter is. That being the case I'd recommend you use Colemak as your main layout but switch back to qwerty for an hour or so every day.

My final speed (about 80-85 words per minute) is probably only slightly faster than my origina qwerty speed, but I have found a massive improvement in both accuracy and comfort. With qwerty I was making typos typically two or three times a word. Nowadays I make typos typically two or three times a sentence.

well, I quit using colemak for a couple days and qwerty started feeling like normal again, that is typing without thinking about typing. now I guess the test is just to see if I can gain that and be faster with colemak. Gonna take some time though, and I am going to alternate between them at least every three days to stay bilayoutal.

If you have both layouts in your history, then it will be easy to switch completely from one to the other with your best speed in that layout.  It will take about 3-4 days of flailing, no more.  What is not going to be easy is to maintain your best speed in both simultaneously.

I was very surprised by this phenomenon: I was expecting rougher weather when I switched from Colemak to Qwerty and then back to Colemak.  Please note I am not talking about learning a completely new layout; you must have known both layouts well once.  It basically means that learning Colemak "cold turkey" does not carry a penalty should you later -- even after many years -- find that you should go back to Qwerty.

From personal experience I typed about 55-60 before in qwerty and when I started colemak seriously I caught up in about a month or so.  I type 60ish in colemak now and 55-60 in qwerty (although there was a period where qwerty went down when I decided not to use it much now I use it 1h+ a day to stay w it).  If you use colemak for about half your typing (don't switch back and forth choose when and stay with it) then you will prob take arround that time.  If you go cold turkey then it would be faster but qwerty will suffer.

Yes sometimes in between you will also have a bit messy for both.  If you try to keep it in blocks of use then it will mostly be transaction time.  I used to take about 5min to get back now I can switch fully in about 30sec  (faster switch but for just the same speed but about that for accuracy).

I type in russian йцукен fluently about 20 years, I type in qwerty more or less about one year and now I type in colemak about 10 days. I think I have quite good progress with colemak. This is a very interesting layout. I try to forget qwerty. It is not so difficult for me because it is not so imprinted layout for my brain.

Yes, I did. Thank you. Of course Rulemak is not optimized for Russian but it can be useful for an english-speaking person who learned Colemak and want to type Russian without trying to learn йцукен. Йцукен is not ideal layout but it is much better than qwerty.

Colemak isn't the best layout for touchscreens, see this thread.

It's a "compose" key, to create a large number of extra (eg. accented) characters.  Read the documentation by long-pressing the hand key > FAQ > More special characters.