graduate admissions - What challenges will an international joint PhD program pose to applicant?

To elaborate, “international” means you have to work at institute A for a minimum of X certain years, and work at institute B for a minimum of Y certain years. They are supposed to be far away from each other geographically (like Europe and U.S.)

And “challenges” are the ones during such kind of PhD life.

NOTE: I also posted this question on Quora.

EDIT: To narrow down the question, I particularly want to learn about the challenges in frequent travelling and working with multiple advisors in different locations.