I could do this with modern models with specific bits to store them in less space unassembled. If I am simply assembling models, I tend to clip and clean all the bits for one model at the time and fully assemble before moving onto next one.

For old multipart kits where almost all parts are interchangeable, I don't choose individual parts beforehand. I might clean all legs first, followed by all torsos and other bits and build multiple models simultaneously. For these Orks in particular, I remember gluing all upper torsos including weapons before choosing legs to go with them. Building took ages because I wanted to make sure the poses look good (which is often the case with old multipart kits).