Quote from econometrics:
you are right.
Most of the time I try to add liquidity on one leg and take on the other. i don't mind paying a 1 cent b-a spread but some stocks trade at 3-4 cents spread which is too wide to take.
I am thinking of something like this:
calculate the cost of taking liquidity on a leg ( (bid-ask spread) * # of shares + ECN fee ) and if both legs cost about the same then go to [Execution 1], if the long leg costs unevenly more then go to [Execution 2], if the short costs a lot more then [execution 3].
[Execution 1]: Enter both legs by adding liquidity, wait for one leg to be executed, enter the remaining leg immediately by taking whatever liquidity is available
[Execution 2]: Enter the long leg at the bid and wait until the entire long leg is executed then enter the short leg by taking liquidity.
[Exeuction 3]: reverse of 2