It's not a case of 'one not doing the job of the other', this is (and more than like at some stage _will_ become) a case of one lead saying "We should do this x way!" while the other says "No, it should be done this way!". With two leads there is no one person (aside from James) who simply says "It'll be done z way". You just don't work this way, at least not for long
As for the numbers of people ... you do not want too many people on this project at the beginning, you can take my word for it or not I really don't mind but I'm confident in what I say, as is Bart and as will I suspect many people who've been involved in group projects of any kind ... start small and grow.