When you trade mean reversion, would you want to require 1 long position to always go with 1 short position, even if one of the two was not the highest probability signal? I wanted to build market neutral portfolios, but found that in reality I'd enter positions according to market conditions. For example, if your ultimate goal is to have up to 10 positions in a portfolio, 5 long and 5 short, you'd enter long positions as markets are oversold, and then enter short positions as markets are overbought. In a lot of cases, it would mean that half of your portfolio is sitting in cash (which is not such a bad thing, as it lowers risk).