As soon as I read the title to your thread, I grasped the depth and intelligence of your thinking. Bravo, man!!!
There is a reason that Tackles are valued so highly and Guards are not. VERY very rare for a Guard to be selected high. Hutchison is the one who comes to mind. And he was so exceptionally singular, Iupati is not at this time considered that level. In fact the debate between Iupati and Pouncey only highlights that fact.
It's relatively easy for a Tackle to covert to Guard (if he can't quite make the grade at tackle) and play at a high level and then still be available to play the tackle position if needed. The same is not true for Guard to Tackle. Guards tend to be ponderous and slower with their feet; a tackle playing guard is more nimble and is a better guard insomuch as he can do more (be a better pulling guard, for instance).
You've judged it correctly IMO. If you pick two tackles,at least ONE should make the grade and at worst you have a decent backup. If you select one tackle and a guard, and the tackle fails, you've really botched it at the greatest position of need on the OL.
It's surprising that so many people don't get it.
Good job, man. You've made me see the draft in a new way. Previously, I had been against selecting offensive linemen with BOTH early picks, but under your scenario, I'm on board.