Unfortunately, I don't see any way to complete both of them, unless there's some glitch that can be taken advantage of, but I don't know of any. Either way, all you get for giving the Muura Gem to the Undead Guild is 15,000 PP and a Necroleech, but it prevents you from advancing in rank in BOTH of the Guilds. The woman who gave you that quest doesn't even offer you any more quests afterwards, so this is obviously the "bad" choice. Sorry, but it looks like only a really weird glitch that might not even exist, or somehow cheating with some hacking device, would be the only way to complete both. So, you're way better off leaving only one quest incomplete by giving it to the Mage Guild, rather than 6 or 7 quests if you give it to the Undead Guild!