Check out those baby & toddler expos, they tend to have a lot of brands represented so you can try out the different buggies. I’m in London so did a lot of research on which double buggies are best for city use (narrower entryways etc) and determined the best tandem double (front/back) would be the cybex gazelle as both seats are big enough to last a while (many doubles have a smaller second seat), and the best side by side is mountain buggy duo. Think bugaboo donkey is also good if you love your fox. That said, we didn’t end up buying any of them! We just got a buggy board for our toddler to sit on. He’s now 2.5 and I’m not sure we’ll ever need a double buggy. I have 2 joolz aers that I bought some off market connectors for so on the off chance we need them both in a buggy, we can snap them together that way. Good luck!
I've got a 15 month old and due in June too. I've gone for the bugaboo donkey 5 duo. It's side by side but can use it as a single pram with side basket or a double which is ideal if the oldest is in nursery etc. I got it to save me using multiple prams! Personally didn't like the idea of one in front of the other mainly incase a nasty animal came up to the pram and went for the one down below. Xx
Oh also John Lewis Oxford Street has several brands you can try, but not sure how many doubles they have
I put an extension on my registry for baby no 2. It's a seat (can convert into a stand ) that attaches to your current stroller