First off, congrats on the new toy... Ok, back to the welding capacity. What's the difference between butt welding .250" plate with no fillet, and butt welding .500" plate with a .250" fillet...? Assuming you fill the fillet I fail to see difference. My uneducated thoughts would consider the filleted weld stronger.
Ok then... so if you have a machine that is only rated for .250" material, then just fillet the .500" plate half way and fill the void. ... or I guess there's the craziness that would come from butt welding the .500" plate other either side....lol
I think dual bevels and welding from each side would be preferred in that case... I get payed to fly people safely from point A to point B.... No one would hire me to weld jack squat. lol