Dear J.,
In my opinion, if she's doing it on purpose, it's just something she's figured out how to do and is doing it.
I figured out how to cross my eyes and did it to my mom all the time. I vividly remember her saying..."I hope you know if you do that long enough your eyes will get stuck that way."
Of course they didn't.
I think what you should do is make it fun for your daughter as a test.
Look up.
Look down.
Look to the left.
Look to the right.
Cross your eyes.
Try to roll them around in circles.
It's actually good to exercise your eyes like that.
If your child actually had an eye problem, such as lazy eye, she wouldn't be able to just cross her eyes at will or look up and down.
My step son had lazy eye in one eye and he couldn't move it to save himself. That was a medical issue.
If your daughter is just experimenting you'll know by having her try to roll her eyes all different directions.
Chances are, she's just goofing around and being a little kid and discovered if she crosses her eyes, she sees two of you when she looks at you. The fascination will wear off.
Best wishes.