Artificial microRNAs (amiRNAs) are the expression products of engineered microRNA (miRNA) genes that efficiently and specifically downregulate RNAs that contain complementary sequences. Transgenic plants expressing high levels of one or more amiRNAs targeting particular sequences in the genomes of some RNA viruses have shown specific resistance to the corresponding virus. This is the case of the Arabidopsis thaliana transgenic line 12-4 expressing a high level of the amiR159-HC-Pro targeting 21 nucleotides in the Turnip mosaic virus (TuMV) (family Potyviridae) cistron coding for the viral RNA-silencing suppressor HC-Pro that is highly resistant to TuMV infection. In this study, we explored the fate of this resistance when the A. thaliana 12-4 plants are challenged with a second virus in addition to TuMV. The A. thaliana 12-4 plants maintained the resistance to TuMV when this virus was co-inoculated with Tobacco mosaic virus, Tobacco rattle virus (TRV), Cucumber mosaic virus (CMV), Turnip yellow mosaic virus, Cauliflower mosaic virus (CaMV), Lettuce mosaic virus, or Plum pox virus. However, when the plants were preinfected with these viruses, TuMV was able to co-infect 12-4 plants preinfected with TRV, CaMV, and, particularly, CMV. Therefore, preinfection by another virus jeopardizes the amiRNA-mediated resistance to TuMV.