Princess is being overly dominant at the nestsite so far this year so if you've seen a single bird, chances are its her. She also likes to try to bite at the nestbox edge, while Trey likes to clean out the nestbox when he gets the chance. The best time to see both is if you catch a feeding - Trey is bringing food right to Princess in the box often these days, it may not be for long, but it happens. If we get warmer days, particularly in the morning if its sunny and not so very windy, you have the best chance of seeing Trey at/on the chicks.
And as they get older and neither Princess nor Trey is big enough to keep the chicks covered, I suspect (just suspect) that that is when Princess will be a little less adamant about handling most the brooding by herself.