I see one problem with this method, the returns of your bet won't equal 100%.
When your odd is 2 on one site (EUR odd) then it's highly unlikely that the opposite odd on the other site will be 2, but more around 1.7 or 1.8.
Lets take an example :
Site 1. Odds you want to bet : 2 - Bet 100
Site 2. Odds you want to bet : 1.70 - Bet 100
If site 1 is good bet, then return is 200 + 30 (30%x100)= +30
If site 2 is good bet, then return is 170 + 30 (30%x100)= +0
So until now it looks great doesn't it ?
But here's the catch, you get 30% of revenues MINUS THEIR CHARGES (bonus to player, taxes, fees etc etc). Now I don't know how much are their charges (not indicated on any sites), but i'm pretty sure that it will cover all your possible wins.
Nice try tho I must admit, I never thought of it and I'm a huge gambler player.