Quick googling shows that in 2021, agriculture produced 10.9 billion tons of emissions, while fossil fuels produced 36.8. There might be some overlap between the two, but I'm assuming not for ease of math.
The way I see it, doing both would be nice, but if we try that, fossil fuel companies and bacon enjoyers are going to end up on the same team, and that's going to be very difficult to fight. The corporations can easily push propaganda like "Fight this legislation, they want you to eat bugs and tofu instead of real meat for your Fourth of July barbecue," and then nothing gets done.
On the other hand, if we target fossil fuels first, we may be able to cut 77%, and by doing so we overcome the "it's too late, there's nothing we can do about it" mindset, and no longer have fossil fuel industries trying to fight us when we target agriculture next. Plus, getting people to drop meat keeps getting easier as meat alternatives get better.
We've got to strategize. Targeting both at once may be the only way we can hit the target, but it's so much less likely. I'd rather stop emissions after missing the target than fail to stop them at all.