To Leash, or not to Leash, That is the Question

Brooke is now walking everywhere.  She's practically running, at least speed walking, and is determined about where she wants to go.  If she doesn't want to go where I'm leading her she will let go of my hand and run the other direction, as she made very clear while we were leaving the Y last week.  Harrison is becoming more confident in his walking everyday.  Another month and he'll be all over the place just like his sister.

Harrison can already throw a tantrum like nobody's business, and with Brooke's growing desire to have things her way, it's only a matter of time before she starts throwing tantrums too.  So do you keep them contained in the double cart, if there's even one available, or do you give them some independence and let them walk?  If you let them walk, how do you keep them close and teach them boundaries? Required hand holding? Leashes?

I know a lot of twin moms who swear by leashes and others who think they're more trouble than they're worth.  If we were going to use them, we'd use ones similar to the Gold Bug backpack harness pictured below.  So for moms out there who have used them before, what are your opinions on them? When did you start using one? Were you able to introduce the use of one successfully, and if so how?  Any input from moms who have used a leash before is greatly appreciated.

On a completely unrelated note, today is one year past H and B's due date.  Happy "adjusted birthday" Harrison and Brooke!