As s/w intimated, it would be much easier/safer to install a negative setback post in the Birdy tube.. As the 35mm Birdy seatpost is open at the top, a simple sleeve and clamp would be all that is needed to modify .. your idea of bolting two round tubes together is not good (IMO) .. normal riding would attempt to shear/twist the bolts.. making the bolts bigger to offset the levering/twisting will weaken the tube more.. the engineer types might comment to say it would work and I'm wrong .. I'd go the easy, safe route that could be undone easily..