Zika virus (ZIKV) and chikungunya virus (CHIKV) are highly pathogenic arthropod-borne viruses that present a serious health threat to humans. Since 2015, both viruses circulate in the same geographical regions of the Americas and are predominantly transmitted by the Yellow Fever mosquito Ae. aegypti. There is a growing number of case reports of ZIKV and CHIKV co-infections in humans, but it is uncertain whether co-infection occurred via single or multiple mosquito bites. Therefore, we infected Ae. aegypti mosquitoes via an infectious blood meal with ZIKV, CHIKV, or both and scored the saliva of (co-)infected mosquitoes 14 days post infection for the presence of ZIKV, CHIKV or both. Ae. aegypti was competent for both viruses with transmission rates up to 73% (ZIKV) and 21% (CHIKV). A substantial proportion of mosquitoes became saliva-positive for both viruses (12%), suggesting that Ae. aegypti can transmit both CHIKV and ZIKV via a single bite. Additionally, co-infections did not influence the infection or transmission rates of either ZIKV or CHIKV. Our results indicate that co-infection of CHIKV and ZIKV can lead to simultaneous transmission by the same mosquito in the field.