Brushify packs can easily be merged together, in the Epic launcher, click 'Add to project' then choose 'Overwrite files'.
This is required because Brushify packs share some generic objects, textures and shaders. Any shared files will simply be overwritten, meaning the files from both packs are merged.
You can do this with as many packs as you like. Brushify is designed so that all the shaders and assets can be merged into one massive project containing everything. The shaders and shared assets can then be used throughout your game and there is no duplication of files.
This is very useful if you're working on a game that requires multiple Brushify Biomes such as an MMO or even just a Shooter or RPG with many varied levels.