pseudorandomization multiple attributes

[blueorange]

Is it possible to limit the maximum number of consecutive stimuli conditions?
For example, is there a way to limit the occurrence of both attributes "aaaa" and "bbbb" showing up?
I tried both of these codes but none of them worked  
"stimulus_pattern": {"order": "pseudorandom",
                            "attribute": "a", "b",
                            "min_N": 1, "max_N": 3}


"stimulus_pattern": {"order": "pseudorandom",
                            "attribute": "a",
                            "attribute": "b",
                            "min_N": 1, "max_N": 3}