Question about DiseasesFollow

I play my DK with a friend, who also plays DK. He is frost and I'm Unholy, and for the most part he is the Tank.

When we start a fight, it is pretty much the same. We both use IT and PS (well I know he uses IT for sure), and then we do our thing. I do my rotation (or attempt, we do kill pretty fast) and he does what he does to keep the hate.

My question is, do my BS and BB (and I have Glyph on Plague Strike, so my PSs to) cause more Damage if the Diseases on the target are from my friend's DK?

I ask because most unholy rotations say to start off with PS. But glyph of PS ups the damage of PS if the target has a Disease on it already. So I usually start with IT (My ghoul is also tied to IT, macro wise. When I use IT he then attacks) and then more onto PS to get the added DMG.

I see.
Thanks.