You can use the Moist Air (MA) domain and set the properties to the gases you'd like to mix. If either species condenses, you can use the "Moist" part of the MA domain for that species. Otherwise, the "dry air" part and the "trace gas" part will be enough. The Moist Air domain is available in R2018a or later.
In R2022a and later, if you have access to Simscape Fluids, there is a 4-species custom domain example for fuel cell system modeling. That model can be repurposed to model a mixture of two different gases.
