There aren't any really good ways to estimate viscosities of mixtures, so this is the best I can do
What happens if you mix two solutions of different viscosities. There are two methods, neither of which has much backing. The first is the Refutas equation. The procedure is to calculate Viscosity Blending Numbers, VBN, for each component, then create a VBN for the mix based on the mass fractions, x i, then convert the mix VBN back to viscosity.
VBN i = 14.534.ln(ln(η+0.8))+10.975
VBN mix = x 1.VBN 1 + x 2.VBN 3
η R = exp(exp((VBN mix-10.975)/14.534))-0.8
The second method is Kendall-Munroe.
η R 1/3 = x 1η 1 1/3 + x 2η 2 1/3
Given that the predicted values can differ greatly, it is hard to know what to do with these values. Just pick whichever seems to work for you.