"Ustedes tienen unas y nosotros tenemos otras."
Translation:You have some and we have others.
Since "unas" is a noun (also being used as a direct object), it does not have to match anything else in the sentence. It only has to match the gender and number of what it is describing.
In this case, you (ustedes) have an undefined number of undefined objects (unas).
Without context, we do not know why the feminine gender is used, but "unas" would still be used, even if the subject was singular (usted).