Dengue, chikungunya and Zika have recently caused large outbreaks in the Americas. Besides those, other arboviruses may have been overlooked. For example, Mayaro virus (MAYV), which was first identified in 1954 in Trinidad, is very rarely detected by Sentinel surveillance in French Guiana (FG). However, we cannot rule out that some MAYV infections may have been misdiagnosed because Mayaro fever shares symptoms with other arboviral infections and there is cross-reactivity between MAYV and chikungunya virus (CHIKV). We propose a statistical approach to assess the circulation of both MAYV and CHIKV in FG using a multiplexed cross-sectional serological survey that includes 2,697 individuals sampled over the FG territory. We account explicitly for the cross-reactivity between MAYV and CHIKV. We find that 41% (95% CI: 35%, 48%) of MAYV cases might be misdiagnosed as CHIKV positive with serology assays. Our framework also allows testing and exploring different scenarios of virus circulation for each region of the territory. We find that MAYV has been circulating for years in the southern part of FG, affecting mostly adult males, and we estimate seroprevalence as high as 27% (95% CI: 19%, 35%) in some regions. On the opposite, a CHIKV epidemic hit FG in 2014, was more present in the northern, coastal regions and infected women more than men.