There are a number of important differences--for instance, I am led to believe that BA assault squads can take meltas, while SM assault squads cannot. (Also reduced scatter, red thirst, etc) I would pay a premium for that.
But essentially no, unless you want to play with an allied contingent from one and a primary detachment from the other, you can't legally get such cheaper models in your 'dex.
Of course, if your opponents in your local gaming store agree to it (or let you include a Stalker, Hunter or Centurions, for instance), we can hardly stop you from doing so. I doubt that many players would let you do as such, but you may as well ask. Don't just go in assuming that you'll be allowed to and not tell an opponent until after you've deployed or something, though.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!