Flavobacterium psychrophilum (Fp), etiological agent of bacterial coldwater disease (BCWD), causes substantial losses in salmonid aquaculture and hatchery facilities worldwide. In addition, reports of previously uncharacterized Flavobacterium and Chryseobacterium spp. (Family Flavobacteriaceae; collectively termed flavobacteria) tied to similarly manifesting disease outbreaks are seemingly increasing on several continents, including North America. In some cases, these “non-Fp” flavobacteria co-occur in diseased fish that are also infected with Fp (i.e., a polymicrobial infection), a matter that complicates disease diagnosis and subsequent control. To date, the effects that such polymicrobial flavobacterial infections have on the outcomes of BCWD epizootics remains unclear. To this end, a study aiming to develop a polymicrobial flavobacterial challenge model was conducted. ~2.5 g rainbow trout (RBT; Oncorhynchus mykiss) were adipose fin-clipped, intramuscularly injected with one of two flavobacteria currently being proposed as novel species (e.g., Flavobacterium sp. S21, F-S21; or Chryseobacterium sp. T28; C-T28) at a high dose (HD) or low dose (LD), and then immersed in suspensions containing either a high or low concentration of an Fp isolate (Fp-US53). During the 65-day experiment, infection status, disease progression, and cumulative percent mortality (CPM) were assessed. Throughout the experiment, no mock-challenged RBT died. In RBT immersion-challenged with Fp-US53 only, CPM reached 10%, compared to ~27% and ~57% in RBT intramuscularly-challenged solely with a HD of F-S21 or C-T28, respectively. When RBT were co-challenged with two flavobacteria, CPM almost always increased. For instance, RBT immersed in a high concentration of Fp-US53 that also received a HD of C-T28 experienced ~73% CPM, possibly suggesting an additive effect between these two flavobacteria. In contrast, RBT immersed in a high concentration of Fp-US53 that also received a HD of F-S21 experienced ~57% CPM, revealing a possible synergism between the two flavobacterial taxa. In the same vein, gross signs of disease were most severe in co-challenged fish when compared to single-pathogen control groups. For example, although F-S21 and Fp-US53 LD control groups were devoid of external pathology and mortality, fish co-challenged with a LD of both flavobacteria developed classical BCWD signs (e.g., deep ulceration of the caudal peduncle), again suggesting some flavobacteria may be working in consortium against their host. When tracking infection status, polymicrobially-challenged RBT harbored active Fp infections longer than fish challenged with Fp alone, a matter potentially affecting the disease course and longevity of BCWD epizootics that involve multiple flavobacteria. In addition to providing a basis for the development of future polymicrobial flavobacterial challenge models, study findings suggest that any potential synergistic virulence between flavobacterial taxa may be species dependent and that some polymicrobial flavobacterial infections have the potential to elevate the risk for future flavobacterial outbreaks in affected facilities.