The reason I dropped Qwerty is that I can always type in Colemak at any Qwerty computer by running a small software. Dvorak users also have DvorakAssist to do so.
Keeping two layouts is possible but totally unnecessary. Counting the time you have to type at another's computer and you decide for yourself whether it is really worth it.
For someone who still wishes to do so you can read a Colemak user's experience in keeping both layouts at the same time. From his experience it is even easier to keep both Qwerty and Colemak since they have 10 keys in common, you only have to learn the rest 17 keys.
http://www.ryanheise.com/colemak/When I first started learning the Colemak keyboard layout, I tried to maintain a skill in both QWERTY and Colemak simultaneously. This was actually not too difficult to do since Colemak keeps many keys in the same place, and the result after 30 days of practice was 104 WPM for Colemak and 95 WPM for QWERTY (down from 104 WPM at the start).
After this little experiment, I discovered a nice way to switch to Colemak on other computers that did not have it installed: simply type in
http://www.ryanheise.com/colemak.exe (provided that computer has Internet access) and the layout will automatically change. For computers without Internet access, you might carry the exe on a USB disk. Now, I do not bother with QWERTY anymore (although it seems my fingers still have the muscle memory).
For those interested in maintaining a skill in both layouts, my strategy was to type in QWERTY for about 5-10 minutes each morning immediately after waking up, and then to switch to Colemak for the rest of the day. You may find it surprisingly easy to switch back and forth.
Tips for learningAfter the plateau at 50 WPM, I decided to pay attention to my weaknesses, and I noticed that my most commonly mistyped character was "G". After simply focusing on "G" as I typed, my speed instantly jumped up into the 60s.
When I got into the 90s, progress was quite slow getting up above 100 WPM. What helped was to practice by typing with a steady rhythm, using the same amount of time to think about each character. This forced me to take the time I needed to catch common mis-presses before they happened, something that I think would have taken much longer to correct otherwise.
Now that my speed is above 100 WPM, my typing technique works something like this: by default, force myself to type at a steady pace -- but, when I see words or strings of words that I know I can type very quickly (e.g. using known finger rolls) then I remember to speed up on those, -- and, when I see words that I know I have problems with, I remember to slow down and type carefully. This, I have found, is the fastest way for me to type, as it tends to eliminate a lot of wasted time introducing and correcting mistakes. However, it requires being aware of your strong and weak words.
