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.

1. Opposite EQ. Use the EQ to carve out spaces for the opposite instrument. Example: Find the fundamental frequency (note) of the Kick, use an EQ to boost that frequency slightly, then use an EQ on the Bass to dip that frequency. How much you boost or cut depends on the instruments and the song. You can also cut other freqs on the kick and boost the same ones on the bass.
2. Mute or lower the volume of either the Kick or Bass when they play at the same time.  This one can be a little more tricky, and it depends on who needs to be heard more. A common way is to put a compressor on the Bass and use the Kick track to trigger the compressor. Essentially side-chaining the Bass with the Kick, so every time the Kick plays, the compressor on the Bass turns it down. Again, how much depends on the instruments and the song.
3. Replace the Kick Drum. Use a program (there are many out there) to either completely replace or mix in with your Kick.
4. Re-tune the Kick. If your Kick is already a sample (not a live recording of your song [no bleed from other instruments]), you can use a pitch-shifting plug-in to shift it out of the way of the Bass.
5. Re-amp the Bass. If you have a direct recording of the Bass (and a bass amp), you can re-amp the bass to better fit with the kick.

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.