I have an Uppababy Vista (not 2015 model). We have the rumble seat too.
We have a range of solutions for transporting toddler (will be 20 months) and newborn in it this summer, including using the car seat and either of them in a sling.
However, the shop mentioned a universal wedge type insert which would make the toddler seat effectively flat. The Uppababy Snug Seat isn't suitable from both apparently - they said from 4-5 months. It doesn't allow a baby to lie flat.
If it makes any difference, I do tend to have big, strong babies. DD was in the toddler seat at 15 weeks and sitting independently at 20 weeks.
Can anyone recommend a suitable wedge?
TIA