With the sport, it is always important to have the heavier child in the middle (i.e. in the toddler seat at the front with the younger child in the main seat or in the main seat with the younger child in double kit at the rear.
it's also important not to have a child in the double kit on the rear, when no child is in the main seat. The unit is very stable, and the centre of balance is well forward, however in this situation the risk of tipping will certainly be greater.
If you want your younger child to sleep while the older child gets in and out, place the buggy in the lie-flat position with the double kit on the front. The younger child can sleep flat while the older child gets in and out of the front seat.
If your younger child has overgrown this configuration and the heavier child leaves the main seat leaving a child at the rear, as always, please hold on to the buggy and place a counterweight at the front.