We need to use to our breadboard an H-bridge with 16 legs
Why?
Go back to the people who set you this problem and demand some education on at least the fundamentals of what you are trying to do. Then maybe if you actually use words that other people can understand, and then you can get help.
I'm wondering it this is actually true
Yes it is the negative temperature coefficient that screws you, the hotter one gets the lower the resistance is so the more current it passes and the hotter it gets. So they don't share current equally.
However this is nothing to do with the original posters request for a bridge with more legs.