This should work just fine, for the most part, though it's not recommended. What would not be fine is multiple PCs accessing the same wallet.dat from, say, dropbox at the same time. The BerkeleyDB format Bitcoin uses for its wallet is not intended to be a multi-user database and as such this can cause problems if both systems attempt an action at the same time.
It should also be noted that if you copy your wallet to a new system and make a transaction it can become out-of-sync with the old copy of the wallet, since Bitcoin generates new addresses to receive the "change" from each transaction by default.
In short, two systems using the same wallet can be done but it's not recommended because of the unexpected results it can produce. If you're simply talking about a backup/restore of the wallet.dat then that's much less troublesome.