This is a squeeze against both opponents. In one suit, you have a threat against one opponent. In another suit, you have a threat against his partner. In a third suit, you have a threat that the opponents share the burden of protecting.
For example:
As South, declarer, you lead the ♣A. To keep the ♥A West throws a spade. After dummy throws the ♥K, East is in a similar position to his partner. To keep the ♦A East too throws a spade, leaving the ♠A-J to win the last two tricks.
Key requirements for a double squeeze are that at least one of the single guarded menaces lies over the stopper (they both do in the above example) and that prior to playing the squeeze card declarer has lost any losers that the defenders are due to take.