Aarrgh! Horrible idea. Imagine two dogs out at the fully-extended leash, jerking behind, in front, and to the side of you? And then, of course, onto the road, when car comes whizzing by - how the heck do you reel TWO dogs in before at least one of them is hit?
Mitzi, my new dog, has only ever used a flexi, and that is about to change - while she may have 'walked nicely' for my cousin, all flexis do is teach them to pull. I will use one in the offleash park with Mitzi, because she is too small to be off leash where eagles catch bunnies, but on the sidewalk she is now learning to walk on a regular leash.