Student or Learner
Is this phrase correct?:" It's possible to cumulate the 2 salaries".
I don't know whether it's correct (you can use a dictionary as well as I can ), but it's certainly not usual. 'Cumulate', although I imagine it can be used as a free-standing word, is much more common in derivatives like 'accumulate' (ad + cumulare = 'to' + 'make into a heap') and 'cumulative'.
The informal way of expressing what you want is something like 'We could roll the two salaries together into one'. Formally I suppose it would be possible to say 'It is possible to conglomerate the two salaries [into one]'. The 'into one' is pleonastic (as it's implied by the verb), but people do it.