How to iterate over all pairs of items from two relationships?

I have a Web API call that will assign one account to one Active Directory group. These requests come in bulk, e.g. add 3 groups to 15 accounts. The request is an Incident object and the groups and accounts are each linked via one-to-many relationships.

So I want to iterate over all pairs (account, group) and issue the Web API call for each pair. However, I can't figure out a way to do this in a one-step. 

The Step-Through-Children action allows me to iterate over one relationship but not both - I can nest another Step-Through-Children action, but the context has changed to the child object.