Multiple sires happen when 2 (or more) sires are bred to a female in the time period her follicles are ready to be fertilized. This can happen when a female retains sperm, and/or gets bred to many different males in the time period given. Resulting in a double, triple, or how ever many sired clutch.
Meaning
a double sired clutch from 1.0 Bee, 1.0 Pin, and 0.1 pastel
you can get normals, pin, spider, pastels, super pastels, lemon blasts, bees, and killer bees.
But i do believe double sired clutches effect the odds. (My personal opinion.)