Hi Ryan,
You can add multiple recommendations criterias or designs (not both) to the same recommendations activity and Recs will give an equal split of traffic to each criteria or design .
It will look something like this depending on what you choose.
It is also documented here.( Create a Recommendations Activity )
If you select multiple criteria, traffic is split evenly between the selected criteria. For example, if you have selected two criteria and your activity is designed to display default content to 20% of activity entrants, then 40% of activity entrants will see the recommendations controlled by each criteria. There is no option to change the percentages for each criteria.
Hope this helps.