If you blend polyethylenes together, the density will be a simple arithmetic average. But, the resultant MI is a calculated value using the logarithmic rule of mixtures approximation and works quite well when good mixing is achieved:

 

log (MIblend) = x log (MIa) + y log (MIb)

 

Where ‘x’ and ‘y’ are the proportions of blend components ‘a’ and ‘b’.
Try blending a 50/50 mix of 1.0 MI and 2.0 MI:
log (MIblend) = x log (MIa) + y log (MIb)
log (MIblend) = .5 log (1.0) + .5 log (2.0)
log (MIblend) =        0          +     .1505
MIblend        =      1.4145

May not have been what you were wanting or expecting!

 

Credit:   L.E.Neilsen, Polymer Rheology, Marcel Dekker, New York (1977)