Whats your budget you are looking to spend on helmets?
I can see your idea of a 2 into 1 helmet but as has been mentioned the chin guard on these style helmets will not withstand a heavy impact the same way a normal full face helmet will.
Unfortunately helmets are one of those things which you pay for what you get, I personally have 4 mountain bike helmets for different types of riding and yes that's probably a bit excessive BUT all have their different uses. If you are mainly going to be doing XC for the moment get an XC lid and then save for a full face when your going to be doing more DH, or vice versa.
Think about something like this Giro Skyline for a basic XC helmet:
http://www.chainreactioncycles.com/Mode ... elID=74192And this No Fear Prime II Evo Full face:
http://www.chainreactioncycles.com/Mode ... elID=57983Both are basic lids but you can get both for just over £70 which is VERY CHEAP!