Doppler:
Doing all one's deco at 20 only "works" when there's 0% inert gas in the deco mix -- that's to say it's 100% oxygen.
Wellll...... sort of... you can't just combine the stops unless you're on 100% oxygen, true, but you can certainly finish deco at 20' (or deeper, for that matter) any time your PPN2 is .8 or less at your final stop depth. (To be completely accurate, you could do it with a PPN2 up to about 1.2, but anything over .8 gets impractical pretty fast)
For example, (I'm using VPM-B, but you'll get similar if not exactly the same results no matter whose algorithm you use) on a dive where the last 3 stops (30/20/10) are 7, 10 and 17 minutes respectively on EAN80, if you do 2 stops with the final one at 20', those stops will be 7 and 28 minutes - an increase of only one minute. If you make your entire deco at 30', that stop will still be 35 minutes total - so if you have high seas, for example, for a penalty of one minute deco time over what's required at 30/20/10, you can complete your entire deco using EAN 80 at 30'.
The same schedule using oxygen would require a stop of 16 minutes at 30' on back gas (in this case I'm using 16/50, but the principle is the same for any typical back gas) and still require 27 minutes at 20' - 8 more minutes deco than required on EAN80 - with no reasonable option to finish deco at 30' for rough weather on that back gas.
Bottom line, EAN80
does work with a final stop at 20 or even 30 feet, with a very small deco time penalty at 20 feet or even 30'. Additionally, there is often a significant advantage (reduced total deco time) when compared to using pure oxygen. And, as an added bonus, using EAN80, even with the final stop at 30', uses less oxygen clock than oxygen with a final stop at 20'.
On the down side, EAN80 is "stroke gas" they say...
Rick