I would personally stick to level stakes [% of bank worked out at the beginning of each day] and forget about staking plans which will only alter your bet amounts, not your SR.

40% POT suggests a very good result but you haven't indicated how consistent that is [over what time period], as most punters would be happy with a consistent 10% POT.

I often bet 2 selections in a race [my wife picks a selection besides mine as a little personal comp. between us]. It works fine as our odds range is 4.50 to about 20.00.
I think odds are the key to betting more than 1 selection regardless of selection method. If I consider the favorite as the top selection, I ignore the race. Betting on shorties can sometimes pay but as 7 out of 10 favorites on average lose, having 2 bets that total very short odds [treated as 1 bet] often means regular odds-on betting or close to it. It might work for some but it's not my cup of tea for profit. If one of your bets is a shortie and gets up at $2.50, you have made 50 cents. Why bother?