Welcome to the Land of Metaphorical Hands.
Essentially, design has stated (though it isn't terribly official) that you can only get "two hands' worth" of utility via two-weapon fighting. You could two-weapon fight with a one-handed weapon (wielded in one hand) and a boot spike, but a two-handed weapon and a boot spike is right out, because design sensibilities and math reasons like such as.
I mean... I get the balance reasons but I will admit that it's kind of annoying since what even is the point of the existence of foot weapons. One could say that they have concealment advantage or something but it's not exactly easy to go and kick someone from concealment like just thrusting with a knife is. also not really related at all but it's stuff like this that makes me wish the clawed races (and possibly classes) could choose to get it on their feet, since generally anyone planning on using a weapon has no purpose for them. Though my opinion on natural attacks being both broken and underpowered (since once you have enough of them they're one of the best ways to go about things, but you have to turn yourself into an eldritch abomination essentially to get the most use out of them) could be a thread in and of itself.