Sq D makes a 'Handle Interlock' , or commonly refered 'Transfere Clip', part # QO2DT1 ( sorry, i could not find a link....)

This is a simple little piece of metal that acts in a see-saw manner when mounted between two 2-P breakers.

Both the latter can not be on @ the same time due to said 'Clip'.

As one of the two breakers may be 'backfed', there is a 'breaker hold down bracket' which is required. I noticed this mentioned inside a SQ D cover's sticker.

Now this entire arrangement might top $15 in parts & seems to be kosher... [Linked Image]
The simplicity and $$$ blows me away...

But I don't see a whole lot of these, as compared to the average T-switch, or Gen-Tran, nor have i installed one....

So i'd like to ask here, are there applicable codes,listings, concerns, etc that i've missed?

Is this a setup simply more common than i've seen?

Unfuzz me please... [Linked Image]