Designrider:
You have to define and explain your problem in a little more detail than you have so far. Some sketches, sizes, dimensions, loads would be helpful in doing this. Normally we look at this problem and assume compatibility btwn. the two beams, in which case they take their share of the load in proportion the their relative stiffnesses, EI1 and EI2. But, Slickdeals suggests they may be stacked on top of each other. And then the question is, are they connected in any way at their faying surface, the flg. to flg. surface btwn them? If they are not properly connected together, there will be a shearing type slippage btwn. them at this faying surface; their load share will be in proportion to their relative stiffness, and the stresses (bending and shear) are figured independently for each beam. When they are properly connected together at their faying surface, through shear flow, they become one combined member with a new moment of inertia, and the stresses are figured accordingly, on that combined member.
Alternatively, you could have the two members side by side and properly connected together, or not connected, except through the load. They will still tend to carry the load in proportion to their relative stiffnesses, but now the stiffness of the load, its C.G., its ability to distribute the load this way without rolling, etc., its ability to force compatibility btwn. the two beams must be brought into play. If they are properly connected together, you might still have a torsional problem, with one side of the new combined beam being weaker than the other, and this has to be accounted for in some way. It relates back to the load rolling problem above.