If two products that should not be purchased together are selected, an error will present to the user as a red text pop-up. 

Event on the Rule Set to trigger the rule: Plan Selected.

IF:

Id = _____ (or Name = _____ )

AND

Selected = True

AND

Second.Id = ______ (or Second.Name = _____ )

AND

Second.selected = True

THEN:

Raise Error (Text entered into the textbox. Does not support HTML. Ex: "Error. These plans cannot be purchased together")