I haven't read all of this so don't know what's been suggested so far.
Firstly, the Duet is great in many ways but I know several people who have had their eldest kicking or pinching their youngest as they're so close together.
I've had a few doubles. We have a Britax b dual at the moment which has many good points but is wide and heavy to lift. However it's one of the very few tandems that's suitable for a newborn and toddler. Many need a separate carrycot, or the spacing doesn't work, or there isn't enough recline for each child etc etc. The B Dual isn't an off roader but we've found it copes well with a bit of mud and roughish terrain. This converts to a single, but makes a very wide single, although the basket is lovely and huge!
The new Jane Twone (out around now) is lighter, narrower and a smaller fold than the B Dual but both seats are only suitable from 6 months so you need a separate carrycot or a Jane Matrix car seat. It's also not an off roader as such but I've found Jane's to be really good off road. This converts to a single too.
We had a Jane Powertrack (similar to but an older version of the powertwin) and we really liked it. The rear seat lies flat or you can use a car seat. The front seat doesn't have much recline but just about ok as long as your toddler isn't really tall. It was so so lovely to push and I really miss how easy it was to use. The fold was also surprisingly small, and smaller than the new powertwin's fold. You can pick them up really cheaply on eBay if you keep a look out.
If you just pool your money to get a convertible single/double then look at the iCandy Peach Blossom 2. It's very expensive though.
I personally don't really like side by sides but if you're looking a them the Out N About Nipper is good and light to push.
There are plenty of other off road side by sides like the Baby Jogger City Elite Double, Baby Jogger City Classic Double, Mountain Buggy Duo, Easywalker Sky Duo, I'm sure there a lots more I can't bring to mind right now too. All of which you could easily get second hand well under £400 but maybe not new. There's also the Baby Jogger City Mini GT but I doubt you'll find that second hand as it's so new.