Facultative root hemiparasites likely connect to several hosts at the same time. However, most studies use single-association experiments, where a host and parasite are grown together in a pot. In this study, we used a split root box which allows us to control both the number of hosts and nutrient supply to each plant independently. We allowed the Phtheirospermum japonicum parasite to connect to zero, one, or two alfalfa host plants, finding a linear relationship between parasite mass and host number. In a follow-up experiment, we tested the effect of parasite N supply on the host – parasite relationship, finding that parasitic damage to the hosts increased with N supply to the parasite.