The TwinGo is a great option for parents of twins or two children of different ages. You can use the two carriers separately to carry individually or they clip together to let you carry one child on your front and one on your back.
Each carrier has a maximum weight limit of 45lb, or a combined weight limit of 70lb when used as a tandem carrier.
Because of the added complexity and safety issues in carrying two children, this is only available for hire after a 90 minute consultation.