Also observe that FL and FR has different bass offloading frequencies. Those two plus the LFE cutoff means that the subwoofer has three different lowpass frequencies here. Sometimes the number can be higher.
The subwoofer will have one correction filter for LFE, another for integration with FL and a third for integration with FR. And since there is only one line for the subwoofer in the crossover tab, there isn't enough room for this info there. But for bass offloading it will always be the mirror image of the high pass frequencies entered: The same frequency and the same XO width as the high pass side.
It could also be done with one sub but it depends on your audio interface hardware configuration. Some interfaces have channels 7/8 on the same output jack. In such a case one jack can "represent" 2 outputs. In AL you could add another channel to the sub it would be termed driver 1 and driver 2. Now it is possible to have HPF and LPF for each driver, just make sure you cross the right drivers. I have tried it and it sounded awful but maybe you will be more successful.