[JoeyP]

Ahh yes this is possession aggression and you my dear are the possession

Being this is coming down to a cat it's going to be hard to stop because going by my two guys .... They HATE cats

Hate is too strong a word ... They just like chasing them
If your goal is to have them both on your lap you need to make your pup see that it's in his or her best interest to have the cat around
Try this
Have the cat on your lap and then have your dog enter the room
If he barks.... Out of the room immediately
Once he's calm start dropping treats to him to get him used to the idea that ... Hey when the cats around I get treats
And slowly bring him clser and closer until you can get them both on your lap

That is .... If the CAT cooperates