It may depend on where you live - do you have a lift? If so, which type will fit into it most easily? Do you have a narrow entrance door? Will you often want to push the pushchair along crowded pavements? Will you often go out alone with both of them together for long periods? If no to the latter, you could carry #2 in a sling or other carrier if it's only for short journeys, or put #2 in the stroller when #1 is happy to walk. By the time your #2 is too heavy to be carried, your #1 may be happier walking for longer.