As we know Chase has a Small Business version for most of its credit card portfolio. If you have a Small Business you sign up for the card and are eligible for the sign up bonus even if you have the Personal Card already.
The best offer for the Chase Mileage Plus Explorer card is 30,000 miles (though it was 50,000 a few months ago).
However the Small Business card still gives you 50,000 when you go through this link.
You will need to spend $2,000 to get the sign up bonus.
While there are reports that Chase has been making it harder to get co-branded cards (i.e. by only approving you for credit if you had less than 5 cards from ANY issuer within the last 24 months) it’s not enforced yet. I have tested that out myself recently.