If you have only one sub it should always be used in Normal mode. In Normal mode, the sub is phase-coherent with the tops when they are facing the same way.

When you add the second sub that's necessary for cardioid mode it does face the rear and it does go in Cardioid mode. The forward-facing sub remains in Normal mode.

The output of the rearward-firing subwoofer in Cardioid mode is used to cancel the wrap-around energy coming from the Normally positioned, Normal-mode sub.

When it's in Cardioid mode, the forward output of the subwoofer is not phase-coherent with the tops, so if you put the sub in its normal, forward-firing position and switch it to Cardioid mode there will be a significant gap in their combined response.

If you'd like to read more see our blog post on the subject:

What Is Cardioid in Subwoofers? (bassboss.com)