When partner makes a take-out double of a minor, and you have both majors, your bid is to CUE BID the minor. This tells partner: "You pick." (Partner cannot guarantee 4-4 in the majors and this guarantees that you will find your 4-4 rather than your 4-3 fit.) After that, you can pass, invite, or bid game. When partner doubles a major and you cue-bid, partner will assume that you are 4-4 in the minors and want him/her to pick—if you had the other major, you would have already bid it.
If, however, partner cue-bids after your take-out double; you bid; and partner then bids a new suit, you are in a GAME FORCE. The cue bid either sets up a "you pick" situation OR creates a game force. Subsequent bidding makes it clear what is going on.) For example, 1♣ Dbl (by partner) Pass 2♣ (by you); Pass 2♠ (by partner) Pass 3♥ by you is a game-forcing bid.