True twin does not mean it is symmetrical, true twin means the tail height is the same as the tip height. Most symetrical skis are also true twins however. so nomensteven was correct.....like he usually is.
A symmetrical ski is....well, symmetrical, could be mounted forwards or backwards technically.
If you buy a symmetrical ski there is absolutely no reason to mount it non center, if you don't want a ski mounted center get a different ski, with all the choices today there is a ski for everything.
Conclusion: MOUNT THEM CENTER.