My aunt boot used to tie them all (nubs) into netting

No seriously, we use Kirk’s Castile soap, when it gets small it goes into a wide mouth gallon container filled with water. It will dissolve down and we use the soapy water for an insect spray on our plants.

Put a weight on it while it dries.

Waste not, want not.

While each bar is only slightly damp. Press the two together, wiggle just a tad while pressing together and then carefully get the bar a bit more wet while turning it over in your hands to create lather. Fill those edges between the two bars with the thick lather, and I mean thick, really thick - then set the bar down and leave it to dry. I usually leave the small bar side up, and then I will be a bit more careful using the bar the next time or two, to make sure they continue to meld into one.
The real key is to create hatching or crosshatching on one side of each bar, and then making some 'slip' to fill in and join the two.
This whole method was adapted from what I learned in pottery class forty years ago, lol.