We have had 2 double prams now. the first we got second hand, and it fell apart after a couple of years! lol. The second, we bought brand new ($400 worth) and it is still going strong. Neither or them have had a tip risk. They are more balanced, because the wheels are at the front and back, so the weight of the pram is distributed over a greater 'floor area'. No sid tip either, as they are a bit like a tank. The one we have now has 'power steering' or so it seems. It is very sensitive to directional change and highly responsive. a great off roader too! lol. Ok, so it sounds like a car- The same principals when buying a pram are used for buying a car.
- Are you comfortable with it? Take the time to push is around for a while and see how well you can handle it.
-Is it easy to get the kids in and out of? If it isn't, then probably going to be more of a hassle than a blessing.
-Does it have 5 point safety harness? If not, then it doesn't meet Australian Standards and your children are going to be come master escape artists.
-Is it in your price range? The lesser known brands produce just as good quality as the top name brands, and for about 1/3 the price as well. We seem to pay a lot for a name these days (don't get me started on that! lol)
-Is it easy to fold down and put back up? Very important if you are on your own with 2 (or more) kids.
-Is it going to fit in your car? Double prams are rather bulky, so you need to ensure before buying one that it fits in the boot of your car. No point buying one that requires the boot to stay open so it gets wet on a rainy day.
-The overall weight of the pram. You need to be able to lift it on your own. An overly heavy pram is not going to be very helpful if you can't lift it into your car.
- Colour? does it really matter? You can always cover it with a blanket if the colour doesn't suit.
Be very careful buying prams from Ebay. You aren't going to be able to take it for a test drive, and they don't exactly have to be honest about the condition of it. The listings that say 'comes as is' are rather scary. For all you know, the wheels could be falling off all the time, which the seller isn't going to state, and because they say 'comes as is' you can't complain about it either.
Your son will be around 3, so why not give him the opportunity to learn to walk? He's going to have to learn at some point, and before a baby comes along might be best. There are harnesses out there (got one here we don't use) that can help with this, and the good ones will be able to hook onto the pram too, freeing up your hands again.