What are you looking to gain by sandwiching two 3030 beams together, over one 3060 beam?

With two 3030's sandwiched, most of the extra metal is concentrated at the center of the web, which adds no real strength in tension or compression to the beam, though you will get a slightly larger cross sectional area.

If you're looking for more mass, two 3030's with the extra mass of the plates and bolts would be the way to go though. Rigidity-wise though, the single 3060 is likely to be stiffer.

Mark