Are IBD and S&T lifestyles compatible??

It's definitely doable - am in the same situation with my girlfriend, and we've made it work for 2+ years now. Not to say that it's always easy, but careers aren't the sole determinant of compatibility. Naturally, whoever's working in IBD has longer hours and may well be called into the office on weekends, etc. As long as the partner working in S&T accepts this fact and is willing to be flexible, it shouldn't prove too much of an obstacle. If anything, both working in finance can be a benefit (especially for those in IBD) - your partner will have a better understanding than most of what your work involves and will likely be more understanding as a result.