Anyone know about property development and can answer a question?

[FiveFootTwoEyesOfBlue]

If a house was divided into two flats decades ago (Victorian house), would the owners of the flats get more money by putting both flats on the market at the same time (linked in some way?) so that a buyer could make it back into one house again? Is that a thing or have I completely imagined it?

It depends on the area, the number of flats and their condition. However, usually a bunch of flats will be worth more than the same building arranged as a house (but not always). Developers are more likely to buy a house and divide into flats than the other way round.
However, if your building is arranged as 2 units and you own the freehold, it might appeal to a BTL investor who might prefer that over flats in buildings they have no control over.

I’d have thought it would depend on the area, eg Chelsea probably someone would want to turn it into one big house! Best to ask a couple of local estate agents.

OK, interesting thanks. Probably a no-go then. It's a desirable road and area of a fashionable city, but nothing like Chelsea. I own one flat, other flat owner is going to sell and I was wondering about selling at the same time.

In my area, it would be quite likely that a house would be worth more but that’s because it’s v v family orientated and there are plenty of purpose built flats anyway.

If your area is more popular with young FTB and trendy etc then flats are likely to be worth a lot more but you could still appeal to BTL investors.