Export private key from MultiBit without password - this is important.Then edit file with some editor like Notepad++ and copy only key - this is row without # ,and you must copy only key - no date and another symbols , and then in BitCoin-Qt open from Help->Debug window ->Console then type

without <>
where label is name of owner - your name.
And when press enter the your address with coins will appear in your addresses and in your balance.My be you can wait until client rescaning the blocks .

In my opinion here no problem to use two different client with the same private key - if you spend or receive the coins from one client with this private key - address it will be synchronized  in the another client - this address - private key.