We've had to think of the same thing....but seeing as our babies will be a little closer in age than your two, we are going for a twin pram, one in front of the other, and will put our 1 year old in the front. We will just use a single pram when we all go out as a family, as my husband will be able to help look after Naomi, and she's basically walking now....so by the time we have two babies, she should be fine to walk a lot of the time with Daddy or Mummy's help.
We did investigate other options though before we decided on a tandem, and you can get toddler seats that attach to a pram...and if you're lucky, you might be able to buy one that fits your existing pram. The toddler seat attaches to the front of the pram and sort of sits up above the other baby.....you wouldn't be able to have both children sitting up properly and seeing out,....but by the time your baby wants to sit up, your toddler will probably not want to be in a pram anyway!
A new toddler seat costs around $70 - $100 depending on what you're getting,...but I have seen them advertised in the trading post and on e-bay. You can also get a jogger pram that has a toddler seat on the front.....the one we looked at was very good, and both children would be able to sit up and see out, but they were over $500 to buy new. I haven't seen any second hand ones of those.
And yes,....you can get a little step with wheels on it that attaches to the back of the pram, and then the toddler can stand up in between you and the pram, but I didn't think my eldest would be able to handle it.....it looks like it would be better suited to a two year old or older, who can handle standing up and hanging on.
So there you go.....sorry I wrote a little essay,....hope it helps.....there are quite a few options out there....have you thought about the fact that you will now need two cots and two high chairs and two car seats and two.......lol!!!!
What fun we are going to have!!!!