I don't have twins but might as well - 12 month old and newborn. I now have three prams so have tried all the options!!

I originally bought a tandem stroller (one in front of the other) for shopping and a double side-by-side valco for walking. I HATE the tandem. It is difficult to manouevre, DD#1 just wasn't comfortable in the front seat, access to the basket underneath is crap. The Valco is absolutely brilliant for both my girls but it is so heavy and big when folded that it just isn't all that convenient to get in and out of the car, and the width is a nightmare for getting around shops.

So, the Valco is still my "walking" pram and I have a P&Ts that lives in the car. TBH I really didn't like the P&T set up for a toddler/newborn so I have used it like you are suggesting - DD#1 is in the pram and I carry DD#2 in an ergo. As of a week ago, I now put DD#2 in the toddler seat but I have an insert that provides her with a fair bit of support, othrwise she would still be too little. FWIW the manufacturers recommend babies be at least 6 months to go into the toddler seat, but with the insert I really think DD#2 is fine. The only thing you will need to check is that you will be able to recline the pram bit enough to keep one baby comfortable without it being too low, and therefore squashing the baby that is down the bottom in the toddler seat.