Question: The three Warhammer 40k RPGs (Dark Heresy, Rogue Trader, Deathwatch) share a somewhat common ruleset, and I believe this question applies to all three.

Can I move through an ally in combat?

Answer: 
The following response was from Mack Martin of Fantasy Flight Games:

> 
> This is a tricky question to answer,
>  as the game isn't distinctly designed
>  with battle mat play in mind. While a
>  grid can certainly be useful in a game
>  (and add some fun strategizing into
>  combat) it would really depend on how
>  much space a single square represents.
>  In my games (and generally speaking)
>  if a square was 3 or more meters then
>  I would declare that you can move
>  through allied squares without issue.
>  If it were less (such as 1) then I
>  would suggest that the character must
>  go around. I would also say that if
>  forced too, a character can squeeze
>  through a space occupied by a friend
>  (if each square is 1 meter) by
>  spending 3 meters of move distance to
>  get through.
> 
> 
> 3 metres per square feels like a lot
>  of space, a 1 meter square, however,
>  might get out of hand if a character
>  can run 18 meters in a round. This is
>  really something the GM and Players
>  will have to figure out on their own.
> 
> 
> None of this is official, obviously,
>  but hopefully these guidelines can
>  help your group come up with some
>  suitable house rules for using a grid
>  mat during your game sessions.
> 
> 
> 

Not a firm answer, but it will have to do.
I'd just treat it as a dodge roll to get by, at the better character's dodge, and count it as 2 squares instead of 1.

Looking in Deathwatch, I can find nothing to indicate otherwise.
However, if one wanted to be thoroughly pedantic, one could declare engagement on the friend, and then disengage, making it a half-move or move followed by a disengage (which includes a half-move).