Hello!
You need to run a trigger that backs in both markets and use a special formula for Amount (the Price is just the default price of the corresponding selection). The formula depends on the particular markets where you want to Dutch, the type of Dutching (back or lay) and the baseline setting, i.e. whether you want a fixed profit from the Dutching or whether you want to limit the liability.
For back-Dutching for a fixed profit of profit and Under 1.5 + Over 2.5, the formula for the Amount field is:
(profit*100.0/(100 - 100*(1/football7_s_1_back_price + 1/football8_s_2_back_price)/(1-commission)))/back_price
See the example trigger attached.