Intensify, escalate (4,2)
I believe the answer is:
step up
I believe this is a double definition.
'intensify' is the first definition.
(synonyms)
'escalate' is the second definition.
(I know that escalate can be written as step up)
(Other definitions for step up that I've seen before include "Increase as with pace - or ascend stair?" , "- Bezukhov (War and Peace)" , "Accelerate - promotion" , "Advancement (on a ladder or career)" , "Approach" .)