I'm a bit perplexed how you would have a single ticket that spans United and Aeromexico considering they're in different alliances, but if you're on a business class ticket you'd have access to the Aeromexico lounge. Also, if you weren't in business class, that Aeromexico lounge is accessible with a Priority Pass membership anyway. If you're an American Express Platinum/Centurion cardholder there are also some Amex lounges you have complimentary access to if there is one in your terminal.