Import two wallet.dat into Bitcoin-Qt client
There is (yet) no multi wallet support in bitcoin-core.

You can try to stop Bitcoin-Qt, replace your wallet.dat (rename the old one, place the new one) and start Bitcoin-Qt. If your wallet is really old, it will perform a rescan for new transactions and this can take some minutes.

To merge your wallets:


Open Console and enter dumpwallet &lt;filename&gt;
Stop Bitcoin-Qt
Replace wallet.dat with your new wallet file (move the current wallet.dat file to a save place)
Start Bitcoin-Qt
Open Console and enter importwallet &lt;filename&gt;.