Dividing my waste is easy enough, and eventually the bin will fill up. Then what? The council will collect my normal rubbish if I put the bin bag outside my front door, but they won't collect my food waste. More specifically, they don't offer any kind of recycling collection to people who live in flats above shops: you can only get a recycling bin if you have a front yard to put it in, and my front door opens straight onto the pavement. That means that I have to take my recycling down to the depot, or to a public recycling bin (e.g. at the supermarket). Unfortunately, they don't accept food waste at any of these facilities. You can collect compost at the depot, but that's no use to me. So, I wind up putting all of my waste into the same bag, and it goes to landfill. That means that there's no real point in separating out my waste within the twin bin, so I might as well just have a normal bin with a single section.
Still, suppose that the council were willing to collect my food waste. According to their website, I'd be issued with a kitchen caddy and an outside container. I would then put my food waste into the kitchen caddy initially, and empty that into the outside container every couple of days. So, I wouldn't actually use the compostable section of my twin bin in that scenario either, and again I'd be better off with a normal bin. Potentially I could use the twin bin instead of the kitchen caddy (to free up space on my worktop), but then I'd still have to store the caddy somewhere (since it's council property).
So, when would the twin bin ever be useful? If I had a garden with a compost heap then I could fill the compostable section of the bin, then throw the entire bag (including bin liner) onto the heap when it was full. However, that's not applicable to me at the moment. Also, because the green bin liners are compostable, that means that I have to replace them every couple of weeks, even if the bin isn't full. Otherwise, the food waste starts to eat through them: I then wind up with a liquid mess inside the plastic bin and I have to rinse that out.
Also, the normal bin liners are easy to buy at a supermarket, but I have to go online to get replacements for the compostable bin liners. There are liners sold in supermarkets for kitchen caddies, but they're the wrong size for my bin. So, if I had a caddy here then it would make sense to use it; if I lived elsewhere, I could empty it onto my hypothetical compost heap.
So, I think there are two lessons for me to learn from this:
1) Don't buy stuff on a whim, with vaguely good intentions. I should make sure that I have a specific plan for using it.
2) Next time I move house, I should try to find out what the council policies are in advance.