If the roo is chocolate and the hen is jubilee...
50% of the pullets will be chocolate partially spangled (with chocolate replacing the black in the feathers), 50% will be partially spangled jubilees
all the males will be partially spangled jubilees (although half of the males will carry one copy of the chocolate gene)
If the roo is jubilee and the hen is chocolate...
all the offspring will be partially spangled jubilees (but half the cockrels will carry the chocolate gene)
so... to create chocolate jubilees... start with a choc roo and jubilee hen... pick the chocolate pullets and breed them back to their brothers (hoping you got lucky and picked at least one brother carrying the chocolate gene), then from those offspring pick the best partially spangled chocolate rooster and repeatedly breed him back to regular jubilee hens.
after several generations of crossing back to regular jubilees (think 4 or 5), it will be possible to improve the partial spangling back to standard millie fleur pattern, but would be chocolate jubilee. IMO, with the mahogany groundcolor, there wouldn't be as much of a stark contrast between it and the chocolate, and the birds would not look as striking as jubilees with black. But I'm one of the few that has never cared for the chocolate color to begin with and never could figure out what all the rage was about it, so the end result would be more one of personal preference. A LOT of work over many years for a product which will likely end up as just 'meh'. It's hard to improve on just the standard jubilee, IMO.