I ran into a situation recently when two of my players were fighting a chaos space marine in melee combat. Both players have double team and one is a Space Wolf. When making attacks, both players get +20 WS to hit from the talent.

I have two questions:

1) Would the PCs get +20 to parry?

2) Would the Space Wolf get +20 to counter attack?

Thank you.

As written, I would say "yes" and "yes" to your inquiries. Ganging Up is a big deal and provides an appropriate WS bonus. You could assume that the Chaos Space Marine has to constantly shift focus between your players, making it hard for him to concentrate and attack/evade either one of them properly.

Double team gives bonuses used with Weapon Skill tests. Any action which requires a Weapon Skill test is therefore improved.

This includes Parry and Counterattack.

The bonus from Double Team is in addition to the normal outnumbering bonus (page 248). In your example the two characters should have recieved a total of +30 to their attacks and parries.