Aim: In the current Anthropocene, many ecosystems are being simultaneously invaded by multiple alien species. Some of these invasive species become more dominant and have greater environmental impacts than others. If two potentially dominant species invade the same area, the combined impact has been reported to be either (1) domination by one species, i.e., the competitive dominance of one invader, or (2) invasion meltdown, where the combined impact is much greater, i.e., a synergistic effect. We studied the effects of the invasion of two alien plant species that are known to strongly decrease native plant species diversity: the Persian walnut Juglans regia and goldenrod Solidago canadensis. Location: We examined native vegetation diversity in abandoned fields (in Poland) where neither species had invaded, only one species had invaded, and both species had invaded. Methods: Field survey data were analysed using generalized linear mixed models and ordination techniques. Results: When goldenrod invaded alone, it caused a larger decrease in species richness and cover (74%) than when walnut invaded alone (58%). ¬When walnut and goldenrod co-occurred in abandoned fields, walnut was dominant and strongly decreased goldenrod density by 87%. However, the combined impact on native species diversity was much lower (15% decrease in native plant diversity) than when either goldenrod or walnut invaded alone. Main conclusions: In contrast to many other studies, our study does not support the occurrence of an invasion meltdown. Instead, our results show that even when one invader dominates, its negative effect on plant diversity can be strongly modified by the presence of another invasive species.