Lol I haven't played NIN/RDM in years!
On a semi serious note, you could give RDM Dual Wield 1 and allow this spell to give Dual Wield 2 and 3 at specific levels of Enhancing. Dual Wield 3 requiring 500+ Enhancing.
Getting dual wield natively means that RDM/WAR becomes very normal overnight. Temper + Double Attack trait means RDM gets 30% Double Attack Natively along with Berserk, Aggressor, and Warcry. Sanguine Blade is no longer the issue for /WAR, it's that RDMs are murderously beating things to death with CDC and Requiescat. All the while, Cure IVs at full potency.
That is a pretty powerful job you are describing. I don't suspect we would ever get a buff again, which means no more magic adjustment. Essentially that's the argument I would have against this kind of change. I'm very pro melee, but I don't neglect my magics, since that's why I'm playing a RDM. If it were a choice between actually good Enspell improvements or native dual wield, I would take Enspells. They are the reason I'm playing RDM to begin with and what I like most about the job. Enfeebles and Stoneskin being my runners up.