Were I trying to work them into a campaign as a decent threat or at least involved in the story, I could make a few jumps to get there:Bah. The demon lord of lubricants.
OK, maybe that isn't necessarily bland, but it's a tricky concept to work into a D&D session nonetheless.
1) Since Juiblix and Zuggtmoy share the same layer, and because their areas of influence are similar, I'd just say they are a duo who work together.
2) Because of #1, I'd look at where their tastes correspond. So, that means Slimes, Oozes, Fungus, Mold (and is Zuggtmoy over disease too?).
What do these things have in common? Well, environment. Molds and fungi exist anywhere that's damp and dark. The best places for that are: Swamps and Underground. Slimes make sense for these areas too; lots of organic matter to munch on, easy to blend into the environment.
3) So seeing their environmental favor, I would say that the duo are trying to either 1) take over these environments, or 2) orchestrate a grand plan to create large swaths of these environments. For 1, you can have the easy "Invasion of the underdark", flooding it with yellow fungus spores and the like. #2 would be engineering a plot to darken the sun (or change its nature), so direct sunlight is eliminated, while increasing rainfall/causing lots of floods all across the material plane.