We have a P&T Vibe and love it. While we don't have 2 children... yet I love the fact that the oldest child is always at the front and the weight is always at the centre as opposed the toddler seats the sit on the front of the pram. But just a thought, if your son isn't into the pram and you are going to use a sling could you use a pram with one of those skate board things fitted for DS to stand on?

DD just woken, will come back if I think of anything else. Good Luck.