Is there a dimmer switch on the market that controls two lights on different circuits? It can have one or two switches but must fit in a single switch box. Also one circuit is a 3 way.

 

Not to my knowledge. Think about it. Two separate systems. How can one switch operate two independent circuits? Your second sentence is possible. Two switches, one each to control each circuit. But physically it won't fit in a single gang box.
It's possible to control a three way circuit (one light two switches) with a dimmer, but the dimmer will have precedence as to the on/off brightness of the light, with the second switch or non-dimming one only making an on-off circuit. There are "companion" dimmers available from some companies, that allow you to place additional dimmers in the circuit. I had asked the same question a little back and if memory serves be correctly, I believe it was PJ who gave me the short answer as a NO.
I suppose two circuits tied to one switch could turn on or off both circuits, but not operate independently.
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