"Why was it necessary to crash the 2nd craft? If they had kept it intact then they could have used it again for another go. Or are they hiding something?"
The 2nd craft traveled with the upper stage the whole way, going the same direction at the same speed, splitting at the last hour or so.
This means that at the end, they were _both_ heading straight for the Moon at about 9,000 kph.
Of course, the point of LCROSS was to get observations of the actual impact, which means it cannot change course; it _must_ be there when the upper stage hits the dirt.
The LCROSS is only 4 minutes behind, at which point it would require a few times its own mass in fuel to avoid collision.
Not to sound patronizing, but look up the Rocket Equation and play with it a bit to see what it really takes to change your velocity by 9,000 kph! Then compare it with how much fuel the thing actually carried...
Anyway, its all irrelevant. Once the upper stage goes in, the LCROSS has nothing left to observe; there can't be a 2nd round because there's no 2nd bullet!
Giving LCROSS the ability to become a lunar satellite after the mission would have radically driven up costs in ways you haven't even thought of.
It would be superfluous because the whole point of the upper stage was to send the LRO (the main lunar sat) into lunar orbit. LCROSS was just a scheme they conjured up to do something with the upper stage instead of throwing it away.
The goal here is cheap, cheap, cheap!