Am In Escrow. Found A Better Deal. How Soon Is It Ok to Offer?

[Mike H.]

I am a buyer. 

I just went into escrow on a place (we'll call it home A). Then I found another home (home B) that is a much better fit. 

Is it ethical to place an offer on B and only cancel A in the event that B accepts? What if there is a short overlap in time that you are in escrow on both before you are able to cancel the contract with A?

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I am in California. No contingencies have been removed. Just trying to do the right thing.