Currently there is nothing stopping Beta users from buying AoE: DE on Steam. You can purchase the bundle of AoE: DE and AoE II: DE and still get the $5 off AoE: DE. It simply won’t charge you for AoE II: DE at this time as you are not able to pre-purchase it currently.
We are working hard on a solution to this issue and will post updates in the beta forums as we have them.