partypooper, I understand what your after just not 100% sure its possible or not as easy to calculate as I may be capable of. The issue lies in that both the stakes would then be dependant on each other somewhat. For example I can make it so that on Horse 1 your profit is $20 and thats fine, it would just mean that you'd have to take your stake on Horse 2 away from that which would reduce the profit below your $20 mark. The problem with then increasing the stake of horse 1 to cover that loss on horse 2, is that you then have to increase the stake on horse 2 to break even, and then your going around in circles. The following is the best I think you can achieve:

Race 1:
Horse 1: Stake $3.64 @ $6.50 to make $20 profit
Horse 2: Stake $1.17 @ $4.10 to break even
If Horse 1 wins total profit is $18.83, If Horse 2 wins you break even, If neither Horse wins go to Race 2

Race 2:
Horse 1: Stake $9.34 @ $5.80 to make $40 profit
Horse 2: Stake $4.29 @ $4.30 to break even
If Horse 1 wins total profit is $35.71 across all bets to date, If Horse 2 wins you break even, If neither Horse wins go to Race 3

Race 3:
Horse 1: Stake $18.67 @ $5.20 to make $60 profit
Horse 2: Stake $8.07 @ $5.60 to break even
If Horse 1 wins total profit is $51.93 across all bets to date, If Horse 2 wins you break even

Thats about as good as i'm aware to be able to get it? Hope that helps? If thats what your after let me know and I'll do up a spreadsheet