We all know that mixing your kick and bass can be a big struggle, especially if you don't use just a simple sub, but rather a full bassline that covers the entire frequency spectrum.
In that case, using a regular sidechain that ducks the entire bassline down, is going to take away a lot of the energy. At the same time, you don't want to duck the bassline just halfway down. That may let some of the high-end through to bring back the energy but also some of the low-end, which in return will mess with your kick drum.
The solution? Multiband sidechain. If we use two different sidechain settings, one for the sub and one for the rest of the bassline, we can ensure that the low-end ducks 100% whereas the rest of the bassline can be ducked, e.g., just 60% instead.
Pro tip: In our example, we used a plug-in called "Shaperbox" by Cableguys as it has multiband sidechain functionality built into its sidechain unit. If you don't own this plug-in, you can quickly achieve the same effect by duplicating your bass, adding a high cut at around 100-200hz to the first bass and adding a low cut at the same frequency to the duplicated bass. Now you can use different sidechain settings on both layers.