I think its a Trochus snail...
Do you know where it was collected - as in what ocean/depth/etc.?
I do not know for certain, but it appears to be a small/baby reef lobster. Most lobsters are scavengers, and will of course make a home in your substrate. They can be very temperamental with their own kind and will show aggression and possibly kill another if they are not paired already. If there is enough food, most will be content with scavenging - if not, they will go find their food (attached or otherwise to a burrowing fish, snail, etc.)
So to answer your questions:
"Is it reef safe?" This depends on your reef. Do you have a lot of burrowing livestock or other small fish/snails/food that it would want to taste? They should be peaceful enough if they have food they can scavenge.
"Should you get the other one?" If they were paired, you have a small window of time before the 'pairing' is lost and they will treat each other with hostility. This is even more of a problem should one become established in your tank and makes a home. Most of the time I find that through the collection process, any real pairs of inverts are no longer paired by the time it reaches the LFS, so I personally would not recommend adding another.
Hope this helps!
