That staghorn has really taken off and is growing into a magnificent specimen, Lin. You have done a good job. You might soon have to consider re-mounting it. Personally, I would hate to divide it because it is so symmetrical and nice-looking. A large, natural cork slab would be a good option. It is very lightweight and you can simply attach that mount/plant right to it without even disturbing the stag. I don't know whether you have a source of large cork mounts, but if you do, look for something in the range of 14"x20" and a bit thicker (1 1/2-2") than the average cork slab. Have the fronds started producing spore-plaques yet?
Even though it is really easy to over-water staghorns in potting media (one advantage of mounting), I still can't explain the way those fronds look.
By the way, and I bet you already knew this, staghorn ferns will literally tell you when they are getting "desperate" for water. The fronds will begin to wrinkle due to dehydration and if water is not supplied, those frond-tips will brown. In fact, lots of my plants will tell me: "Water me now!"
