I think runearm with bastard sword or daxe should beat out dual wielding tbh - the optimal split just based on reading ddo wiki seems to be tier 5 in BE (spending 41 ap for the capstone if pure, 34 ap if multiclassed), with tier 4 in AT giving access to blast rod sla for just 23 ap spent (though you'd probably spend a further 4 for the +1 Int and +1 spell crits. With this set up, you get the full power of a runearm in addition to the main hand weapon, and I think the runearm > offhand weapon damage. With TWF, you lose out on the additional spell power from the runearm (seems you can get +40 with charge at tier 5), making the runearm+1h weapon combo even more enticing.
Level 15 arty still seems worthwhile to me, for access to deadly weapons. If you're WF, level 16 arty (or perhaps Level 15 arty + bladeforged as they get reconstruct as an SLA) is needed I think.