SPA Modeler

mostrich wrote:That screams after an "it depends..."-answer. Should I stop now?

Some planes (most of Messerschmitts' were) are build semi shell style. So there is a small seam on the real one that let you as modeller get away with yours.
If that is not the case the best way is to melt (or weld) it all together. Use Tamiya Extra Thin or that more aggressive stuff called Tenax (hard to get over here in old Europe), let it do its work in the seams and when the plastic is softened press both halfes together until material squeezes out. After curing just sand it flush and voilà, a perfect one part fuselage.
The main advantage here is the dealing with only one material and no filler at all. Makes sanding much easier.

Hope that helps.

Merry Christmas,
Torben