Assuming everything is set up correctly, let me suggest that you give Mark Johnson a call at Precision Tandems near Kansas City. He's no stranger to troubleshooting unusual problems and upgrades and, given that you're already looking at the FSA MegaEXO cranks & bottom brackets, he'd be able to make a sound recommendation on what the latest available configurations would work with your desired gearing.

Phone: 913-962-8866 between 10:00 a.m. to 9:00 p.m. CT
Email: info2007@precisiontandems.com