The latter: you're substituting STR for DEX, not stacking both stats together. Same goes for other PrE abilities. Sorry, but DDO isn't
that generous with the power creep.
The closest you can get to "double-stacking" is a STR+CHA-based build with Divine Might; or a <dmg stat>+INT-based build with Harper using Know the Angles which provides an Insightful bonus to dmg equal to half your INT mod. Note that an INT-based build with Harper can have both INT to dmg from Strategic Combat (or other sources like Targeting Sights) and the dmg bonus from KtA; so essentially INT stacks with itself in this case.
Caveat: KtA doesn't stack with Insightful Deadly gear, just as Divine Might doesn't stack with Insightful STR gear. So bear that in mind when planning your gear lists.
It's doable, of course, but it's also a flavor build choice, insofar as you're giving up a lot of melee DPS by not sticking with either pure rogue or MCing with, say, monk for Henshin bonuses (e.g., rog 18 / monk 2 or rog 12 / monk 6). You get some useful buffs and utility spells from wiz 7 like Haste, Displacement, and Dimension Door; but those can usually be UMDed from scrolls just as well by a pure rogue. [DDoor scrolls are rare drops, but it's not like you need it for every quest, either.]
That said, it's up to you whether you're willing to trade DPS for extra utility / flavor. Somewhere I posted a halfling rog 13 / wiz 7 vampire staff build for someone who was running in an all-undead static group.