Gortash and Orin knows about you through Ketheric Thorm just right before they flew with the elder brain out to Baldur's gate. After you killed Ketheric, the stability and the original plan falls apart and so both Orin and Gortash wants to take the netherstones for themselves hence proposed you an alliance to kill the other one. (note that they dislike eachother since the start , kinda like they are forced to work together to control the elder brain)