If there is a need for two separate services at two different (albeit close by ) locations they will use two discrete ‘lines’ from the pole with the CBT (fibre distribution point ) one to each location .
Rather than extending the existing fibre an extra 20m the obvious thing to do is leave that existing ‘line’ as it is and arrange for the new user to take over that service ( you cease service , they order service themselves and become the customer on that ‘line’ ) and you arrange a brand new service at the location 20 metres away .
Generally, the network is designed around 1 ‘connection’ per dwelling, so there could be a network capacity issue if both addresses are not recorded correctly , is the outbuilding a registered address in its own right , you can check what Openreach have by entering both the addresses here , if both are not shown separately , getting a second service may be tricky
https://www.broadbandchecker.btwholesale.com/#/ADSL/AddressHome
As far as your questions
a/ …Yes they can extend a fibre , but they won’t .
b/ Yes they do have a way to use a single fibre for more that one service but it’s not FTTP portfolio product , you can’t order two services on the single existing fibre .
