When we had DS1 he was in the middle. When DS2 came along they were on both passenger sides. When DD arrived i had all seats installed professonally and he recommended that the capsule go on the passenger side, DS2 is in the middle and DS1 in his booster + harness is behind the driver. It is easier in our situation to have the capsule on the side, it really can get quite heavy but having the two boys side by side can get a bit crazy if they're moody and start agrivating eachother.