Are you using the same wallet on two different machines?  If you are using the same actual wallet.dat file you might be safe. 

However if you are using different wallet.dat files the two wallets will become different as they would be generating new addresses which are different.  They would have started off as the same wallet but when bitcoin-qt adds addresses to the wallets it will be different on the two machines which means they will start having different addresses. 

If you are wanting to use the same wallet on different machines you should use a deterministic wallet such as Armory.