Not too useful but thought I'd add anyway, the two-tone 70's Firebird Formulas, as an example, were sprayed entirely, then masked up, probably sanded, and the lower band of colour sprayed straight over. Each vehicle had its own unique overspray underneath, and masking technique. as well as different angles of front and rear fender lines. There was no total clear coat as these also used a flat paint for the lower section so that was that. Having said all that, when restoring mine, which still hasn't occurred, they also gave me the 2 options you mentioned. Spray paint the lot, then come back and add the extra colour, then sand and clear coat the lot. Otherwise, try the half-half approach and try to blend in the join. Or do either and no final clear coat over both, just walk away as had 2 sheens. I'm not sure if this also applied to any with different roof paint, giving a satin finish or gloss like the rest, to look more like a vinyl top rather than paint.
On these it was covered by a pinstripe anyway, whereas other models it would be a decor moulding like a fin flash or roof panel to body. I'm not sure they had a preference, but I remember discussing that one would be far more professional, probably the clearing of the lot once done, but also agreed that the other would be more original. Kinda factory vs custom paint, sometimes the best way is not the suitable way, depends on what the finish is. Anyway, good to see how others have done it.
https://www.schmitt.com/wp-content/u...-FORMULA01.jpg