Yes, multiple vehicles can be linked to a single purchase invoice by creating the purchase invoice first, rather than after adding a vehicle and creating the purchase invoice from the Stock & Value section.
To do so simply go to the Sales -> Create Purchase section of your MotorDesk account.
Then add the vehicles as necessary, issue the invoice and then either mark as paid, or go to the payments section and select 'Add Vehicles Now'. The multiple vehicles will then be added and linked accordingly to the one purchase invoice.