I've never user it, but the OOTB example process:
Issue Review and Escalation - Step Check Project Management Office has a condition of:
( project.thisProject.obj_stakeholder2 == null )
as well as
( Project.thisProject Project Management Office != null )
You may want to review to see if you can use this format.
Note, you won't be able to 'generate' it using the GUI section of the 'Condition Builder', and you will have to cut/paste required code into the 'Expression' section.
In your case it would be something like:
( project.thisProject.attr1 == null ) && ( project.thisProject.attr2 == null ) --> Action Item Step
else -- Next Step of Process
Note, obj_stakeholder2 is a look-up but hoping that the above will also work on a string attribute.