If you want to get another rod, then a 6wt would be a good complement to the 8wt. Everything you can do with a 6wt, you can do with an 8wt. However the converse it not necessarily true.

I fished the surf for seven or eight years with an 8wt before I tried a 6wt. I still use a 7 or 8 much more than the 6wt, saving the 6wt when the waves are small.