If you are playing with a CPU pet then this is very simple. On the first level of the first campaign just stay in the drivers seat and direct your pet towards turrets and/or shield. Don't get out of the drivers seat for anything and your pet won't cross you. The achievement will pop once you exit the level through the heart.
If you are playing with a 2nd human player then you might find this achievement more difficult as the ships have a lot of ladders and pathways in which you cross each other and your human friend may just do their own thing. So the best way to go about this is to rank up to rank #5 and purchase the "Banana Split" Ship. To get to rank 5 I believe you need to rescue 60 captured friends. With the Banana Split ship the players cannot cross into each others stations, one player has control of the right side and the other of the left side with the drivers seat and map being in the middle.