Japanese allergic subjects are commonly sensitized to both house dust mite (HDM) and Japanese cedar pollen (JCP) and combined treatment with sublingual immunotherapy (SLIT) tablets is desirable. However, mixing extracts of two non-homologous allergens may compromise allergen stability and affect the clinical outcome. Therefore, we investigated the stability of major allergens and total allergenic reactivity of HDM and JCP SLIT-tablets following dissolution in human saliva or artificial gastric juice. Two fast-dissolving freeze-dried SLIT-tablets were completely dissolved and incubated at 37 °C. Major allergen concentrations and total allergenic reactivity were measured. After mixing and co-incubation of HDM and JCP SLIT tablets in human saliva for 10 min at 37°C, there were no statistically significant changes in major allergen concentrations. In addition, no loss of allergenic reactivity of the mixed two SLIT-tablet solutions was seen. In contrast, complete loss of allergenic reactivity and detectable major allergen concentrations occurred when the two SLIT-tablets were dissolved and incubated in artificial gastric juice. These results demonstrate that HDM or JCP major allergens and the total allergenic reactivity of both SLIT-tablets measured here remain intact after dissolution and co-incubation in human saliva, supporting the possibility of a dual HDM and JCP SLIT-tablet administration regimen if clinically indicated. The complete loss of allergenic reactivity after incubation in artificial gastric juice can furthermore be taken to indicate that the immunological activity of the allergen extracts contained in the two SLIT-tablets is likely to be lost or severely compromised upon swallowing.