A user is only able to use multiple titles on a job/order if it is turned on at category level. To set this up go to Employer > Click Client > Categories > Category Rules on the relevant category and ensure it is turned on. If it is not turned on you will need to select multiple jobs/orders and define the number of jobs/orders titles allowed and click save.
If you cannot see any reference to multiple job/order titles or how to turn it on, raise a ticket to the helpdesk team who will have it switched on for your site.