I need to be able to create an address in 2 different Bitcoin-qt instances (actually 2 different PCs).
I can create it using "getnewaddress" on the first one but how can I import it to the second one with the private key?
Check this section out: https://en.bitcoin.it/wiki/How_to_import_private_keys#Using_bitcoind
Essentially you can use
$ bitcoind help [...] importprivkey <bitcoinprivkey> [label]