Oh yeah, I always use up a bar of soap down to a sliver, then press that sliver into the next bar of soap.
You have to get the old sliver pretty small or it won't "stick" to the new bar.
It's an art form
After living frugally for all of our lives, me and the wife have recently decided that we can now throw the micro thin soap slivers away. Since we are retired and living below our our conservative withdrawal rate, we have decided is our new single nod to "extravagance."