It's not really that easy avoiding seam lines.
In regards to Unwrapping, you could have a better result if you use pelt in this situation after stitching the two pieces together (see unwrapping a character's face for a better understanding of how pelt works). That way, you can avoid seam lines on crucial areas of your object.
If you don't want to do the above mentioned action, it's about your texturing skill. You'll have to manually paint along the seams in your texture editing software such as Photoshop or Substance Painter or whichever application you use to texture.
