If they are brushed motors, you can simply wire them in parallel to the output of a single speed control. Bear in mind that this ESC needs to have TWICE the normal rating, because two motors means twice the current going out to them. If, for example, one motor drew 10 amps, the ESC would need to be rated for at least 20.

If you already have two ESC's you can connect things with a Y-cable, as you suggested, but I think you have to remove the red wire (whichever one carries +5v) from one of the receiver connectors on one of the speed controls. The receiver cable carries power to the radio from the BEC (battery eliminator circuit) in the speed control, and you don't need twice the normal power to the radio- just to the motors!

Steve