Yes they are one in the same, and since its still that people would complain that the "singleplayer" Is too short if it lasts under 8 hours. I was trying to make a guess based on that. Yes the missions will have more of a replayability due to them being made to work as multiplayer as well but it still has to be long enough to fulfill a singleplayer experiance.
2 things have already been mentioned by the devs:
1. Playing through both campaigns should take around 10 hours
2. Brink is designed as sort of a "gateway drug" to MP gaming.
With this in mind, you cannot use a "not long enough for singleplayer" mindset, since it is not designed around traditional singleplayer mechanics. Even the singleplayer is designed to have a high replay value, so that players will want to play it again and again, so the campaigns really don't need to be that long, since no matter how you play it, it's all the same.
Typically, for the vast majority of players, this is how it will play out:
Buy Brink
Play both story campaigns, just to find out the story (and get achievements)
Play nothing but MP, co-op matches, or solo bot matches.