"The two needles are hers."
Translation:Cele două ace sunt ale ei.
You need to use "cele" here because of "două". It works the same way with other numerals too: cei/cele trei, patru, cinci... If there were no numbers, then you should inflet "ace".
A good song to remember this rule that I would recommend since it's entertaining and comedic is Cele Două Cuvinte by Taxi ;)