dolly do you happen to have a maxicosi carseat?
if so you can buy an adaptor and put baby in carseat on top and DD in other seat underneath.
SGK said she saw one in a shop local to her for £5!
might be worth considering!
If not, then recline the back seat and get a small footmuff, or buy a cheap head hugger.
they do look a bit uncomfy if they arent big in the back!
i find its an awkward time when they get to 5 months for a P&T, BUT stick with it cause in a few months it will be perfect when she can sit up and fits in better!!