Here are Thor's Olmo and my Ron Cooper being packed into a single bike transport box prior to Eroica Gaiole 2015. Note that the third wheel shown (and the missing fourth wheel) didn't make it into the box so were packed separately. We somehow did manage to get the lid closed with the two frames (with one fork removed), all components, two wheels, and a number of tools. I used some lengths of allthread, washers and nuts to keep the dropouts from being squished together. The downside of this is that the box adds a lot of weight and so the total two bike package was very heavy. I don't think I'd do that again...would probably do the allthread, the pipe insulation, and bike bags.
That said, most of the plane travel I have done for tours has involved getting a cardboard box from the LBC and cutting it down to the smallest size possible that could take my bike. Again, pipe insulation and allthread axle spacers are critical items.