The dual-hatted individuals must always act in accordance with the rules and interests of the body that called the meeting. At a council meeting, their interest in the charity is non-pecuniary, so the rules depend upon the wording of your standing orders and code of conduct regarding participation in meetings where they hold a non-pec interest. The wording of a dispensation may also be relevant.
In an ideal world, you would aim for a situation whereby the members of the council who are not personally involved in the charity are sufficient in number to be quorate in any decision without the involvement of the charity trustees, so the trustees could speak to make their case, but not vote on a resolution. This provides clarity for all concerned.