I don't think that there is a rule that only one negotiation method may be used at a time. Third parties use integrative negotiation along with other methods. It happens especially when there is more than two parties involved.
I think it shows the mediator's negotiation skills too. If the negotiations have reached a halt because it is not possible to resolve one issue with distributive negotiation, then, the third party can jump to a different method like integrative negotiation to get past the problem.