+ your existing coins are on both forks.
+ your copy of MultiBit will automatically follow the fork with the most work done in it which, according to the current Bitcoin rules, is the main chain.
+ when there are two competing forks you want to be careful on sending product out even if your transaction confirms (explained below).
When there are two competing forks you can have a transaction that is confirmed on ForkA but is, say, unconfirmed, on ForkB.
If ForkB becomes the longer chain then the transaction on ForkB becomes the 'true' transaction.
We did some consultancy work for an exchange and recommended that they wrote special fork monitoring code. When a serious fork is detected we recommended they just 'press the big red button' and stop any fiat redemptions until it was resolved.