Splitting as a standee

Hello folks

In the UK playing as a standee, ie side bets on other peoples hands, is very common. In this situation if the player decides to split, the standee can double his bet too or put his original bet on just one of the hands.

This choice would presumably give an extra advantage as even when BS says to split - the hands can still have a negative EV, in these situations it would pay to not put any more money down on the table.

I was wondering if anyone had any figures on this and specifically whether the gain from the pairs issue outweighs the loss from having someone else play the hands who isnt counting ( assuming they do at least play solid BS )