Trying to make a 2 wide block

I'm trying to learn how to make a 2 wide block, I have the two models for the blocks in game already and just need to figure out how to make them into one block, like I want when I place the "main" block it also places the secondary block that's supposed to be right next to it. I tried looking at the door, bed and tall grass block files to try and get a gist of how they're set up but couldn't figure much out from either of those. I apologize if this is poorly worded, but if anyone could push me in the right direction that would be appreciated!