Two friends of mine, Alice and Bob, enrolled in two different Ph.D. programs, have been collaborating in a research project and writing a paper together. They have submitted the abstract to a certain conference and hope to publish the paper in the future (the conference allows this). Alice, expecting to graduate sooner than Bob will, asked Bob by email if she could include the paper into her "staple" dissertation. Bob is completely fine with that, but he is confused at the same time, since her email was lengthy for the seemingly simple favor. He thinks that perhaps the favor would have consequences according to some customs in the academia; he is worried that the decision might make it impossible to use the paper as part of his own dissertation.
Question: Can a single co-authored paper be part of two stapled dissertations (by two people)?
(Alice is in the Netherlands, and Bob is in the U.S. They both are enrolled in interdisciplinary programs, but the closest fields include mathematics and theoretical computer science.)