As a DPS, I see two relics that I like - Devastating Vengeance & Serendipitous Assault. Can I double up the serendipitous assault to get twice as many power boosts, or does the cooldown time of one also prevent the other from working?

You can't double up the proc/clicky relics for extra boosts, they do share a cooldown.

 

Most dps run with focused retribution and serendipitious assault for level 75 content.

 

For the level synced content (99% of the pve content) those 2 relics are useless since mastery and power are capped. DPS usually run with the devastating vengeance and the alacricity clicky relic for that content. Using the clicky relic can be annoying though, since it makes the level synced rotations different from the level 75 rotations.