Is it possible to mix two types of admixture in in a concrete mix, example; a retarder and a super plasticizer?

Yes it possible to mix two plasticizers and mixing of two plasticizers are commonly used now a days. In the mix proportioning of pumping concrete super-platicizer, pumping aid admixture, air entrapment and retarder are commonly mixed according to the requirement.

As specified by you, retarder and super-plasticizers are now available in a single admixture. You can use that too.