I also converted from a twin seat to a set of single captains seats. As has been said it’s easier to convert down as the single seat only uses 4 of the 6 fixings – which are captive nuts set into the floor pan. To go the other way you will need to add 2 fixings (which probably won’t be there) with reinforcing plates etc under the floor.
You will also need to check out the passenger airbag situation as going from a single passenger to a double may require a different set up. Mines a 2004 so doesn’t even have a passenger airbag anyway. But for insurance purposes you need to get it right.
I sold my double seat for £50 on eBay – which is what they seem to go for. Also DO make sure you get one with a folding back (rather than a fixed one) as these are much more useful & sought after.
The doubles are pretty uncomfortable for passengers though – particularly on long journeys – so I would make sure you try one out first….