I suggested to rainman last week that both at once with more options would possibly work well.
Depends if he wants to max out the accessories/alternate outfits for both or just the standard suits really.
If just the suits I say both at once, clothing and headsculpts sets so people can choose, reg fullsets of each and then a dx double pack around $1250 for both.
Or separate releases and really max out on clothing and accessories.
Vincent Vega:
Full hit man suit with alternate jacket and tie set up. Alternate sculpt with ponytail from diner scene. Gun, briefcase and packet of red apples.
Jules Winnfield:
Full hitman set up, gun, big kahuna burger, BMF wallet.
Or you could also do alternate bloody sculpts and alternate 'dork' clothing for both.