are you looking for a suitable viscometric / rheometric technique, or a general rule of calculation that you can use to calculate the viscosity?
If your materials were of one polymer then the viscositlies just add proportionally to their volume fraction, but if different polymers there may be some interactions that would throw that off.
In terms of viscometer / rheometer, you could use a rotational or capillary system to do this.
Please feel free to contact me if you want more help.