Users can have more then one Memsource account as long as each account has a unique Username.
Translators can work for several agencies as their Linguists and have their own Team Start edition account as well.
But it is important to always use the relevant Username and password, since only the correct Username can access specific jobs.
Sometimes the best solution is to have only a Team Start account and act like a Vendor - ask the agency to share the projects or jobs.
Please note that you cannot use your Linguist account for translating other jobs - only those assigned to you by a Project Manager.