We report on a combined in-beam PET and prompt-gamma Compton imaging system aimed at ion-range verification in proton-therapy treatments. A proof-of-concept experiment was carried out at the radiobiology beam line of the CNA cyclotron facility using a set of two synchronous Compton imagers and different target materials. The time structure of the 18 MeV proton beam was shaped with a series of beam-on and beam-off intervals, thereby mimicking a pulsed proton beam on a long time scale. During beam-on Compton imaging was performed utilizing the high energy γ-rays promptly emitted from the nuclear reactions in the target. In the course of the beam-off intervals in-situ positron-emission tomography was accomplished with the same imagers using the β + decay of short-lived activated nuclei. The targets used were stacks of different materials covering also various proton ranges and energies. The experimental results obtained in this work are compared with a Monte Carlo model of the experimental setup. The results demonstrate the possibility to combine both imaging techniques in a concomitant way, where high-efficiency Compton imaging is complemented with the high spatial accuracy of PET. Empowered by these results we discuss a new methodology for enhanced accuracy and real-time ion-range monitoring. We suggest that a pulsed beam with a suitable duty cycle, in conjunction with in-situ Compton- and PET-imaging may help to attain both real-time and high-accuracy range monitoring.