I came across this sentence in the TV-series 2 Broke Girls, S06E13 00:12:48. But I don't get it.
And the context is:
"Here you are. You two wander off more than my stepson's left eye. You're late for your scene."
It's a joke. It's not a common expression.
There are two things referred to as "wandering eye". First is a medical condition in which one eye doesn't point towards the object that the person is focussing on. (The medical term is intermittant strabismus)
The second is a metaphor for someone who looks at women other than their wife/girlfriend, or someone who looks at women's bodies while talking to them. (Or it could also be someone who looks at other men)
In the quote "Here you are." means "I have found you here", rather than the expression used when giving something to someone.
The rest is funny because it is rather shocking that someone would make a simile like "wander as much as (someone's) eye". Don't use this in everyday conversation.