I don't really understand what you mean with the 2 bets part, but calculating the correct odds is simple.

You deduct the odd you see on Betfair by 1, then subtract the percentage of your commission rate off that number, and then add the 1 again.

So for an odd of 2 at Betfair, you first subtract 1, which gives you "1". You then deduct your commission off of that, so 1 - 5% makes is 0.95. You then add 1 to that number again. So your odd would be 1.95 in this case.

If you bet multiple units at different odds, you need to calculate the weighed average of your odd. Here is a simple calculator to do that:

http://www.rapidtables.com/calc/math...calculator.htm
Betfair also does this automatically for your if you click the "average odd" button just below your bet stake.