In my pot design, if I don't use a soundboard, I don't get a breakover in the yelp. I know this from gluing calls up turned for a soundboard, but simply forgot to put them in.... DOH!
As for double sided calls, the ones I have turned use the soundboard for the under-surface of the call. I have yet to get a consistent breaking yelp from the under-surface of a call. I'm certainly not saying it can't be done, just that mine don't. I really prefer to focus on the calls that I know work, work well and work well consistently. I'll turn a double sided call, but I will tell the buyer that he will get single note yelps from the under-surface.
Hope that helps...