Something very similar has been proposed many times before, as a fix for T4 and during the T5 playtest (there were an awful lot of people against the T4+ task system).
What's interesting is that, although the multi-dice does have more range than the 11 results from the 2D, the multi-dice aren't as wide-ranging as one would expect. Realizing this was how I figured out how to crack the 2D puzzle.
The more dice thrown, the flatter the distribution. With many dice, you end up with several in the middle. There's not as much spread with the dice in the middle.
With the 2D conversion, one data point is used for several that are clustered together with the multi dice T5 system. That's why the 2D system can deliver results that are within 10% of the T6 system. When you throw 6D, there are 31 possible data points. When you throw 2D, there are only 11 possible data points. But the 2D results can be spread across the results of the 6D distribution because the 6D distribution has most of the data points clustered around the midpoint.
Take a look at the dice charts on pages 26-27. See the mid-point on the 6D table? Results 20, 21, and 22 are all the same. A 9% chance.
So, those results are all duplicate.
Then, the spread from there is very small. If you roll 23, that's just a 1% difference from rolling 22. Rolling 24 is just a 1% difference from rolling 23.
Here's the breakdown on 6D
6-12 are all a less than 1% chance. 6 is just as likely, for all practical purposes, as rolling a 12. These seven data points could be represented by a single number.
13-14 are both at the 2% mark. Both could be represented by a single number.
20-22 are at the 9% mark. All three could be represented by a single number.
28-29 are at the 2% mark. Those could be a single number.
30-36 are all less than 1%. Those seven could all be a single number.
MY POINT
If you roll 2D, you get 11 results, and all 11 are practical results.
If you roll 6D, you get 31 results, but 21 of those results can be combined into just 5 results.
Meaning: Rolling 6D only delivers 15 practical results.
Therefore, the increased range you get from rolling multi-dice really isn't as wide as one would expect.