I've heard of using bridal veil too but never bothered. Even if the rubber band causes it to split down the middle it will fuse back into a single colony in time as it grows. All of the toadstool frags that I make and sell have done that. To the buyer it just looks like they get two frags on the same disk for the price of one. Or what you could do is just remove the rubber band after a week or two as soon as the coral has attached on its own to minimize the splitting.
There are lots of other ideas in this article which might help
Securing Propagated Corals
http://www.reefkeeping.com/issues/20...ture/index.php