I do buy 2 at a time once a year. 2 front ones. The backs last about 3 years. Why would one take half worn tyres and stick them on the back? As I said, next time round you've to get 4 tyres.
I bought 4 new tyres last January + tracking cost 380, used to be a Deuce 10 years ago !
Last week the front were more worn than the back but all still good fer use,
(passed NCT), so at my service last week I rotated back to front and front to back + tracking so in another few months all tyres will be equally worn and replace them all at the same time, saving off-road time in the garage replacing them one or two at a go.