1. In a large bowl, mix orange juice, lime juice, tequila, oil, chipotle purée, and salt. Reserve 1/2 cup of the mixture. Place each chicken breast half between 2 sheets of waxed paper or plastic wrap; with a flat mallet or a rolling pin, pound until breast is an even 3/4 inch thick. Peel off waxed paper or plastic.
2. Place chicken in bowl with marinade; turn to coat. Cover and chill at least 30 minutes or up to 4 hours (see notes).
3. Slice onions 3/4 inch thick. Rinse, stem, and seed peppers; cut into eighths lengthwise. Brush vegetables generously with reserved orange-juice mixture.
4. Set vegetables on a barbecue grill over a solid bed of medium coals or medium-high heat on a gas grill (you can hold your hand at grill level only 4 to 5 seconds); close lid on gas grill. Cook, turning as needed, until vegetables are browned and tender when pierced, 8 to 12 minutes total. Transfer to a platter as cooked.
5. Lift chicken from marinade (discard used marinade) and lay on grill; close lid on gas grill. Cook, turning chicken as needed to brown evenly, until no longer pink in the center (cut to test), 6 to 10 minutes total. Transfer to a board and let stand until cool enough to handle, then slice across the grain 1/2 inch thick.
6. To assemble fajitas, lay tortillas on grill as needed, turning quickly, just until warm, 3 minutes total. Fold a warm tortilla around chicken, peppers, and onion; top with avocado-orange salsa.
To make chipotle chile purée, whirl the contents of a 7-ounce can of chipotle chiles (including sauce) in a blender or food processor until smooth. Scrape into a plastic container and store airtight in the refrigerator for up to 3 weeks.