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.