Ok here is the problem I see with this.
Bear in mind that I'm trying to use real world concepts here, not game mechanics or fantasy concepts.
I have actually held a flamberge in my hands, I have an idea of what it's like, and I'm 6' foot even.
I've also swung a chain, but not the length of the spiked chain in the books.
A two handed flamberge would be carried not sheathed, if one wanted to use it. If you strapped it to your back as it was common used, you couldn't draw it. Chances are, you were moving from one place to another, so once you got to where you were going, you took it off. This wouldn't be problem with Landshknet's as they usually knew exactly when they would need them. And it would be in an open field combat.
So let's say you are carrying the sword in your off hand, chances are to give that hand support, the flat of the blade is resting on your shoulder.
Even if you are very tall, chances are that close to half the blade is going to extend upward past your head. (it might perpendicular to your head, but you get the idea.
To get the chain to damage, you need to get it up to speed. I thought that a spiked chain needed to be used two handed, but maybe I'm wrong. Even still, just using it one handed would imply that you would need to be able to swing it over your head without anything blocking it.
Trying to get the chain over the blade of the sword would be extremely awkward. In my opinion, I wouldn't allow for this.