Thread: New bench & benchtops question

Hi all,
First time post to the forum and, as seems commonly the case, it's about the first stage of workshop development: building a workbench. I have been dabbling in woodwork for many years but haven't had a place to do it until now. I have been doing some renovations on my new house and now having finished most of that I have turned my attention to more furniture related activities. I have settled on a design for the bench along the lines of a Chris Schwartz roubo style bench and should cost me absolutely nothing in timber and vices. I even have a bench top already in my possession to use. It is actually two old lab bench tops that were to be thrown out and are approx 1800x750x40mm finger jointed and laminated pine (prob P. radiata). I rescued them after they were removed from the first year biology labs at ANU that were being renovated. Each one is very solid and heavy and I was going to use both (one on top of the other to give 80mm) for the bench top. The rest of the bench will be M&T jointed recycled old construction hardwood that I have pulled out of builders skips and de-nailed in the last year. Straightening that stuff enough to use is hard work though.
An interesting aside was that I actually collected these benchtops about 5 years ago and thought that one day I may be able to use them. I moved them between two share houses and eventually got sick of moving them around so left them behind in my second last share house. As I needed a bench in my new house, I went back on a whim and knocked on the door, explained to the current tenant what I was after and my connection to the place and retrieved them from the old chook shed where I had left them 4 years previously. They were covered in accumulated possum and fur but otherwise as I left them.

The bench tops are sealed (probably polyurethane) on the upper side and maybe single coated on the under side but the difference in sealant has meant one of the bench tops is slightly cupped across the width. The other is much flatter. My question for the forum members is how to attach the two tops together and attach to the bench legs? I was going to use tenons on the leg tops at least into the bottom benchtop to attach to the legs but I am wondering if I should bolt the two tops together with the upper one concave side down before flattening the top or try to eliminated the cupping beforehand? Or use adhesive top join the two tops? Or screw the top one to the bottom? And/or extend the tenons into both to pin them together? Also would it be worth sealing the undersides to avoid further cupping. For some reason the undersides have 4-5 roughly parallel circular saw grooves cut into them I have no idea what purpose they served. The cupping may be related to their presence allowing differential expansion in the timber although the different levels of sealing top and bottom no doubt helped as well as the possum juice and sitting in the extreme temps under a steel roof in Canberra.

Grateful for any suggestions. Sorry no photo yet. I will document the WIP project though.