I think the X by 0.5 applies to the first persons half, the purchase of the remaining 50% would be a CAT issue rather than a CGT one at the point of sale.
Assuming there was no loss or gain then 100% of the selling price would qualify for PPR relief minus the years it was rented out, but there is also a rule where the last 12 months of ownership get classified as a PPR even if you didn't live there so don't forget that aspect of it. Can't recall how the acquisition and selling costs are treated, I think they are apportioned (anybody know that?)