Arbusers

  Considering that we want to make a combo value bet against pinny, where the pinny odds look like :

match  odd  margin
  1          a        X
  2          b        Y

the question is what margins do we have to take into consideration if we want a value combo bet between these 2 selections. X x Y?  X + Y ? (X + Y)/2  ?

Thanks.

It depends on what you mean by margin.
Generally: With decimal odds a, the impied probability is 1/a. So for a combo bet for two independent games, you should expect odds of 1/( (1/a) * (1/b) ).
Do that for the fair odds (no margin) and by comparing with the actual odds you can evaluate your edge in the combo bet.