An increasing production and using of nanoproducts results in releasing and dispersing nanoparticles (NPs) in the environment. Being released into various environment components, NPs may interact with numerous pollutants, including other NPs. This research aimed at assessing toxicity of combined binary mixtures of NPs. The study focused on assessing mixtures of NPs believed to be toxic (nano-ZnO + nano-CuO) and nano-ZnO/nano-CuO with the ones that are insignificantly toxic or non-toxic NPs (nano-TiO2/nano-Cr2O3/nano-Fe2O3). Toxicity of combined mixtures proved comparable to toxicity of individual mixtures of NPs (the sum of effects triggered by individual types of NPs comprising respective mixtures). Toxicity evaluation was based on two parameters: seed germination and inhibition of root growth with respect to four plant species: Lepidium sativum, Linum utisassimmum, Cucumis sativus and Triticum aestivum. The findings showed combined mixtures of NPs to be significantly less toxic in comparison to individual mixtures, irrespective of their components. Within the scope of concentrations used, greatest differences between the toxicity of mixtures were reported at the 100 mg L−1 concentration. Toxicity levels of combined and individual mixtures might have been determined by a lower total concentration of Zn and Cu metals and a greater aggregation of particles in combined mixtures than in individual mixtures.