A showcase for some neat double doors.
Open 1 Door and the other opens automatically.

There are 9 designs in total in here.
4 are kinda the main ones and another 4 interesting ones.
plus my first working concept of it (it's very slow and not reliable)

Edit: added 3 new designs
One inspired by ahorse2 on reddit
https://www.reddit.com/r/redstone/comments/fh0wk3/fast_and_seamless_double_doors/
One i tweaked a design by Bonicgamer to work with 1.15
https://imgur.com/psSHNZ2
and the third a hybrid between the 2

Video Showcase
https://www.reddit.com/r/Minecraft/comments/fdqmj6/double_doors/

Video for the 3 new designs
https://www.reddit.com/r/Minecraft/comments/fibo1z/automatic_double_doors_hipster_fastinstant_fast/