Matters on your weapon speeds. I'll give the short answer version;
Optimal setup is DW 2 "slow" weapons (something like >2.50), and have WF on both.
If you don't have 2 "slow" weapons, and are running something like a "slow/Fast (<1.50)" DW combo, then the OH buff is a tad more "situational".
Spell dmg scales up the frost/flame tongue dmg, so "techincally" if you had more spell dmg you'll improve it. But, gearing to maximize FT/FB dmg is a moot point as a Enh. So that being said, against "high armored" targets I'd tend to buff OH with FT (since magic dmg would be unmitigated), and FB for PVP (easier to chase down targets when it procs, obviously). I'd tend to use RB against targets that are "strong" againt the elements that FT and FB cover, its kinda a good "default" OH buff.
In many ways trying to optimize OH buffs that isn't WF, its really splitting hairs IMO. Goal is to try and get the 2 slow DW weapons and do WF on both. Course I understand because of weapon options @ a given lvl, this isn't always possible, but that should always be your target goal when choosing gear setups as a DW Enh sham.
Regarding buff placement. Assuming both your weapons are "clean" (buff free), the first buff you click on to place will always go to your MH. The 2nd shoudld always go to your OH. Once these are active they are persistant to that given weapon until unchecked or they run their duration. For example; if you have WF on MH and RB on OH, reclicking RB or WF will reset the timer on that given buff for that weapon. If you try to "change" the type of buff to weapons that currently have a buff on them, then it will give you the "highlighted" hand to you choose which weapon you wish to reprioritize.
Personally, if I'm changing up weapons buffs (something I don't do very often I might add since I setup for the slow/slow goal), I just unclick all the current weapon buffs, and reapply from a clean slate. Its easier :p
Edited, Oct 2nd 2007 10:16pm by gpyfb