The application of attenuated viruses has been widely practiced for protecting crops from infection by related severe strains of the same species. Papaya ringspot virus W-type (PRSV W) and zucchini yellow mosaic virus (ZYMV) devastate cucurbits worldwide. However, the prevailing of these two viruses in cucurbits cannot be prevented by only one protective virus. In this study, we disclosed that co-infection of horn melon plants by two mild strains, PRSV P-type (PRSV P) HA5-1 and ZYMV-ZAC, a previously developed mild mutant of ZYMV, confers concurrent protection against PRSV P-type and ZYMV. Consequently, mild mutants of PRSV W were created by site-directed mutagenesis through modifications of the pathogenicity motifs FRNK and PD in helper component-protease (HC-Pro). A stable PRSV W mutant WAC (PRSV-WAC) with R181I and D397N mutations in HC-Pro was generated, inducing mild mottling, followed by symptomless recovery in cucurbits. Horn melon plants pre-infected by PRSV-WAC and ZYMV-ZAC showed no apparent interference on viral accumulation with no synergistic effects on symptoms. Agroinfiltration assay of mixed HC-Pros of WACHC-Pro+ ZACHC-Pro revealed no additive effect of RNA silencing suppression. PRSV-WAC or ZYMV-ZAC alone only antagonized a severe strain of homologous virus, while co-infection with these two mild strains provided complete protection against both PRSV W and ZYMV. Similar results were reproduced in muskmelon and watermelon plants, indicating the feasibility of a two-in-one vaccine for concurrent control of PRSV W and ZYMV in cucurbits.