I've never understood the concern people have when starting opposing pitchers, especially when both are capable of having great games.

You have zero chance of getting 2 wins, but you have a much greater chance of getting at least one win, and a much less chance of getting zero wins. Also, their ERA, WHIP, and Ks are independent of their opponent.

If they weren't facing each other, you'd have a greater chance of getting 2 wins of course, but you'd have a less chance of getting one win, and a greater chance of getting zero wins.

Logically, it makes no sense not to start both just because they are facing each other.

I know in Football you wouldn't start two defenses against each other, but didn't know about in baseball with starter

Not always the best approach in football either. If I happen to have the #1 and #2 defenses and they are playing each other, 9 out of 10 times I'm going to start both. The only time I wouldn't do this is when I needed points so badly that I needed to gamble on a less capable defense in order to gamble that I'd get two shutouts. It's a gamble tho because both D's could collapse, and the one defense I didn't start could excel.

Bottom line:

1) starting best players/teams == best chance of getting points period

2) avoiding starting opposing players/teams == gamble for more points, risk losing more points