This does exactly as the name suggests! It allows opening more than one door at a time, making double doors behave as you might expect that they should. This works for both manually opening them as well as activation with buttons/pressure plates/etc.
You can bypass this functionality by holding the sneak key and opening the door with an empty hand.
This mod requires the Meddle mod loader, as well as the DynamicMappings and MeddleAPI mods. It's easy to install, just look here for more info, and feel free to ask if you need help. This mod just goes in your 'meddle' folder with any other mods.
Since it uses DynamicMappings, it should work automatically with each snapshot. Though due to the number of changes made to the game during development, DynamicMappings will likely have to be updated along the way. Keep an eye on the Meddle thread to stay up to date.
This mod requires no client-side mod to be present. That means you can run a Meddle server and vanilla clients will be able to take advantage of the functionality. It's still recommended for clients to install it as well though to avoid the tiny delay you might see when manually opening doors.
Just an FYI: 15w43a changed the door sound IDs for some reason, so you'll want to grab the update for this mod and the new DynamicMappings. It's for the best anyway because now this mod includes its required mappings list so that it can fail more gracefully in future updates if necessary.