Is the charging port on the iPhone 6s plus compatible with that of the iphone 6s

They both use a Lightning connector. Any cable that you would use to charge the 6s will also work to charge the 6s Plus.

I don't know if I'd say they're "compatible" since there's really no way to connect two iPhones directly. However, they are both using the same Lightning port form factor.

If you want to change parts of two phones - don‘t. Don’t repair phones yourself. Exchanging parts can be very dangerous.