2 is more than twice the work of 1 in the earlier years, but makes it easier later on (although probably not easier than just 1), as they play (and fight) together.
1 didn’t make too much difference to my riding, but 2 did – needed twice as many ‘riding brownie points’ as it is harder work for Mrs STS. Nowadays they’re a bit older so the brownie share price is back to normal.
All good though. More fun with two IMHO
edit: true, 1 -> 2 is greater work than 0 -> 1