[catabur]

With Dual-Blade, you're not able to put two of the same blades on it.

For instance, when trying to put two Huge Blades on the Dual-Blade, it just says "That does not seem to be a valid slot for this item part."

It almost makes me think that this was intentional, but it seems kinda silly when you think about it.

[Sawrock]

You actually can. Just finish putting the first one on there, close out the prompt, and re-try. It's possible to get a 4-range sword with such (and a Giant Gene).