video + photography
Professionals know how to work together. If they are both on the same wavelength and don't come in with competitive agendas. Which I have seen. Naturally there is a better chance that there won't be problems if they have worked together before, but again there shouldn't be if your choices are professional and mature. One reason to possibly separate them is you MIGHT get better service and quality from an independent rather than in-house. Often the video included in a photography package is very basic and with someone who works FOR the photography studio rather than WITH. They might treat you as their own customer and give that little (or alot?) extra if you hire someone separately.
