Question about Crafted Weapons with Undead Damage Bonus% while dual wielding

I see people using Hand Crossbows similarly to get the armor pen bonus, so I was assuming it worked for either weapon but what im not sure about is whether you can stack them? ex Flanged Mace / Short Sword on a fighter. Would you get BOTH 15% Undead Damage Bonus, if so is it calculated *!5% -> *15%bonus or *30%bonus. tyx for any help