Cross tying your horse is only important if you want it to be important. Ignore those at your barn who tell you it is. It's really none of their business.
Cross tying bothers quite a few horses. And once they've broken a tie, it will be difficult (not impossible) to get them to accept it again. One of the problems with a cross tie is it's just too restrictive. There are a lot of good horseman out there who will not cross tie their horse.
I wouldn't worry about it unless you really want to train your horse to do this. But since your horse ties with one tie just fine, personally, I'd let it alone. Don't let other riders intimidate you into believing this is one of those things that a horse must be trained to do. IT just ain't so.
If you feel a need to do this, one way to correct it is to construct a tie system using rubber tire tubes (strong ones) and attach them to your cross tie. Put two strong halters on your horse. Your horse will most likely fight it, might even go down on the ground, but after a while, as long as the rubber doesn't break, you'll have won the battle on the cross tie. Then you can ease your way into using the cross tie normally. Keep in mind, if your horse is clever, he just might break the tie again without the rubber and two halters. Horses have a damn good memory.
But, is this that important to you? Why is it that important? If you can answer both questions to your satisfaction, than go get the tire tubes. If not, let it be.
cya,
Howard (the worst thing about boarding your horse is all the experts telling ya what ya should be doing with him)