In the context of interferometric integral field spectroscopy at the LBT, we have investigated the physics of coupling light from a double pupil into optical fibers. Previous fiber-fed astronomical instruments solved the problem of coupling light from a single dish into single- and multi-mode fibers. However, this is an unexplored territory for a telescope configuration such as the LBT. We have investigated the coupling of the telescope's double pupil to optical fibers in detail, and present results from numerical simulations and preliminary measurements from an experimental setup.