Not a bad idea at all. You should know that every once in a while double streets go into phases of repeats in a small swarm. Also, two double streets can be a hot dozen. You will see hot dozens streak several times in 200 spins. So, I would suggest being aware of these situations and side step them if you can. I like the fact that you have found a good way to just bet on 12.
DS's do go into frequent repeats, the problem is you never know in advance which ones will go ahead of the pack. By the time you notice it, it's usually too late.
However there is a way to take advantage of the very few DS's that will form longer streaks.
For every number spun you bet on its double street. ( you also dot the DS on a score card layout so you keep track of the bets). Then you bet the next number on its double street while rebeting all previous DS's. (YOU DON'T REMOVE ANY CHIPS FROM THE NEXT BETS- IT'S LIKE PRESSING REBET AND THEN ADDING THE NEW DS SPUN). When the same DS hits, you put another chip on it. You also bet all the other DS's that have spun. YOU DON'T REMOVE ANYTHING. Eventually those DS's that hit more often, will pay with ever increasing chips.
If you end up having to bet all 6 DS's, you remove equal amounts of chips from every DS until you have simplify them all.
As an example in the pictures, taking the numbers framed with blue marker (21 to 35), and dot them on a blank layout card, you will notice that DS's 25-30, 13-18, and to a lesser extend 31-36, are way ahead of the rest. Hitting them with an additional chip each time pays big.
You can either follow this procedure for the long run, or start from the beginning after you show a profit, (always simplifying when all 6 DSs have appeared). That way you take advantage of new streaks, when older streaks will eventually begin to disappear.
If you have too many chips accumulated on the DS's you can place a chip on O.
The same concept works on single streets.