How to remove dark lines between static meshes?

I have a dark lines between two static meshes, why, how can i remove them?

Hello GhostyGhost2015,

What you are seeing is a bleeding of your light maps. This can cause artifacts like the ones that I see in your screenshot. I have a couple of questions and troubleshoots for you in order to resolve this.

1.) These appear to be BSP’s that have been converted to Static Mesh’s. By default, your light maps remain the same as when they were BSP’s. The resolution, when converted, is set to 4. This is typically quite low. You can try setting this to a resolution of 64 or 128 and notice a difference in how well your light maps are calculated.

2.) When you build lighting is your lighting quality set to Production? If not try this.

3.) Is there a special reason you need 2 Static Meshes to make a wall? If you only need a Wall then perhaps using one Static mesh would be a valid solution. You could also create an additive BSP. Then create a slightly smaller BSP, whose top is higher than the first, and change this to subtractive. This will effectively hollow out the inside of the BSP. However, using this method, after hollowing out the first BSP and converting to a Static Mesh this will fill in the hollowed out area.

Thanks, it helps, and, im using two because its the two different blueprints.