Hello,
In the plan you have laid out you describe two tenons coming together and butting against each other mid way in a through mortice but you leave no room to accommodate for shrinkage and expansion of the post. The effect will be that the tenons bottoming out against one another will force a gap at each shoulder half the width of the amount of shrinkage or if pinned possibly even worse.
Your thinking on the pegs is correct and with that in mind but also the extra effort involved in the peg openings, the octagonal or six sided peg in a round hole is the best compromise solution.
Greetings,
Don Wagstaff
