You're playing with direct increase[add] skill items so the same script rule is sorta applied in this and the damage goes as this - you do 200, 4 bow guardian give +200% so you triple your final dmg and 200 becomes 600.
Your dmg*[1+modifiers] = final dmg
200*[1+2] = final dmg
200*3 = final dmg
600 = final dmg
If you use something as 2 tg and 2 bow guardian the math would be something +/- like this:
[Your dmg*[1+bow guard modifiers]]*[1+tg modifiers] = final dmg
[200*[1+1]]*[1+0.4] = final dmg
400*1.4 = final dmg
560 = final dmg
[at least the game and calcs show me this ._. but I may be wrong ofc so if someone see that this is wrong correct me]
And if someone is wondering, yes, this is also how it seems that sword guardian cards work and any other direct increase skill dmg.