I think it would still make a strong enough joint if the boards were jointed to fit, the glue should be plenty strong. But if you’re really worried why not try a double tenon joint with one on each side of the glue line?

Keep it simple. Mortise, through-tenon, wedged ends.

I’ve mortised joined boards in the past, for example two 4 x 2s to make a 4 x 4, and never had a failure. My current bench was made this way over 20 years ago – still going strong.

Modern glue joints are often stronger than the surrounding wood itself. When joining boards, make sure that each of the mating surfaces are dead flat for their full length end to end, well glued and well clamped.

If you centre your tenon on the joined side, mortise down the middle of the joint, you are still finally gluing the tenon sides in contact with each of the two halves of the main board; a through tenon on a side-rail at right angles to this, that is wedged in the end, will bond both halves from the opposite direction.

Good-fitting joints + Good glue + Good clamping = a strong joint.
It won’t fail.

Joining boards like this is quite common…

Very often the price of thicker timbers is often more expensive and increases with size. For example, 4 inch thick Oak in the UK is often more than twice the price of 2 inch material, for many commercial reasons, basically because there is less demand for it. Add to that the fact that drying takes much more effort and time which also adds to the cost. Even then, acceptably dry but thicker materials contain stresses that are released when you cut into them resulting in boards twisting and more waste as you try to rectify it.

Combining boards together as you have can add to strength and stability. Consider a rope, made of lots of small twisted fibres, that is stronger in total than the sum of its parts. Translate that into wood and you end up with laminated material, even if it is only two large boards, as in your case, they are laminated. Joined well they are very strong and permanent – last for years.

The next thing to consider about the frame of your bench – after strength – is size and move-ability. You can hire in beef if you need strength, but can you get it through standard doors in one piece if you move house or workshop?

Good luck……… why not post a few pictures when it is complete?

Thank you.
That’s some very fine timber – the sort that we don’t get here in Wales, with some very unusual spalting patterns in that centre board.

On the subject of benches, have you seen ‘The Workbench Book’ by Scott Landis?
It came out about 30 years ago, published in the Us by Taunton Press…….. should be still available if you search it out. Excellent research and bench projects of all types.

Good luck.