Note: this is from the perspective of someone who identifies as genderfluid, and is completely subjective, as is anything trying to define identities.
This is actually going to be rather short, as this is a fairly simple difference.
Genderfluid people’s gender changes with time, no matter how fast or slow, and no matter what or how many genders it goes to.
Bigender people are two genders, and it may change with time.
Genderfluid and bigender can be very similar. The key distinctions are that genderfluid people’s gender is changing in all instances, and bigender people’s gender is two genders in all instances. (bigender people, please correct me if I’m wrong about this)
In my case, I’m genderfluid, but with that, I’m bigender at some times. For me, when I’m bigender, I feel male and female simultaneously. (There is really no way to explain how it feels.) People who identify as bigender can feel two genders simultaneously like that, or drift between the two, similarly to genderfluidity.
Hopefully, I explained this well. Leave any comments you might have in the comments section, including any corrections you may have. I’m Ari, they/them.