Here you can see a solid gutter cover.  Where the 2 pieces of gutter come together there is always going to be a lot of extra water at the valley.  Some companies put in a cheap and easy solution like the one in the picture.  This isn’t going to catch much water.

At Capital Seamless Gutters we use a manufactured High Flow Inside Mitre Panel.  This actually extends past the gutter giving the panel a lot of surface area to slow the water down and push it into the gutter.  The panel is screwed to the gutter and secured tightly under the shingles giving it a lot of strength.

 Gutter Guards Madison