It’s best to start with a great arrangement, parts that work together, great players, and great instruments. However, as a mixer, sometimes, these conditions are not always in place when we get our hands on the song.
The two main things I look for when getting the Kick and Bass to work well together are Timing (are they playing at the same time?). Frequency (are they competing for the same low frequencies?).
Usually, for the Kick and Bass to play nice together, they CANNOT both be playing at the same time and competing for the same frequencies (playing the same note). If they are, here are a couple of things you can try.
These are common remedies for helping the Kick and Bass play nice together. However, the best way is to have an arrangement/parts that naturally work together or address the frequency issue during the recording process by tuning the Kick to complement the Bass or using a Bass amp setting that makes room for the Kick.