I've seen in the past that several agencies specialising in Asian TS girls offer duos with females, but the only one I can find at the moment is this:
External Link/Members OnlyI've never booked with any of those agencies because I prefer independent girls.
In 25 years of punting shemales as well as regular girls I've only had one such duo, and that was years ago in Newcastle. It was tremendous fun, but the two participants were Geordie best friends and we all had a real laugh together.
If I were you I'd find a well-reviewed TS you find attractive and see her alone, going just as far as you're comfortable with the first time. If you see a mixed duo you still won't know for sure that you're into shemales because it might be the prescence of the female that gets you aroused.