Hi Validator
As a general rule, the twin engine takeoff power limit will be to protect the gearbox, it may not be capable of absorbing the combined power of both engines at more than this power setting. Either engine will probably be very capable of producing more than this power continuously, if fitted in a single engine helicopter for instance or if the other engine fails.
TC's comment that you can keep 're-applying' the 2 minute or 30 second power is incorrect - for instance on the EC135 (with TM engines) the engine is designed to provide three bursts of 30 second power in their lifetime (if memory serves me correctly).
Cheers
TeeS