Jim and William are purchasing a dwelling jointly which is intended to be their main residence. They each already own a dwelling, which they (separately) used as a main residence. Neither old main residence was sold at the time of purchase.
Upon purchase, as they will each be deemed to own three dwellings and they will not (yet) have replaced their main residence, the ADS will apply. Jim then sells his previous main residence 5 months later. But they will not yet be able to claim a repayment of ADS, as the joint buyers have not replaced their main residence. William then sells his old house 13 months later. At this point, Jim and William will be eligible to claim a repayment of the ADS they paid.
It will not be possible for Jim and William to amend the original LBTT Return made for their next main residence transaction because the 12 month amendment period will have ended. Jim and William or their solicitor can, however, write to Revenue Scotland to make a claim for the repayment of the ADS paid. Any claim would need to be made within 5 years of the filing date of their original LBTT Return.