Can a task under RACI matrix be accountable by 2 resources?
E.g.: Both the Project Manager and the Development Team Head are accountable for the project quality.
No, it can't. In the RACI format, the Accountable person is the one ultimately answerable for the correct completion of the task. It is also the one who delegates the work to those marked as "Responsible".
As such, there has to be only one accountable specified.