User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-4823600-20130209152029/@comment-1359322-20130209161235

Ok i think you're misunderstanding something, Issei doesn't have 2 Longinus he only has one that is the Boosted Gear, he only acquired the Divine Dividing power by forcefully inserting the jewel on the DD Scale Mail so it doesn't count as a Longinus. And the part with the magic power is a complete misunderstanding on your part. First Vali never used any magic power, he uses his Demonic power. Second, the Divine Dividing does not use lifespan just by summoning it, it is the Juggernaut Drive that does.

Also Azazel mentioned that since Ddraig and Albion are of opposite nature, using the Dividing Gear too much might damage the ability of the Boosted Gear so he teels him not to use it.