SLF3
Below V1 it must be OK or an engine failure would result in you going off the runway.
I assume by "it" you mean being able to control the yaw/swing, control flight path etc with the good engine still producing lots of thrust whilst below V1??.
Apologies if I'm teaching you to suck eggs and apologies to the purists but simple answer follows .
It's assumed in Boeing, Airbus et.al. procedures that if you have an engine failure below V1 you'll immediately select idle on the (other) good engine PDQ, you
don't leave it running at take-off power - because (and there are other reasons) you've got to get rid of the high level of asymmetric thrust - if that is sustained you may well indeed go off the runway....
The reason for this is that below a certain speed ( known as Vmcg, which varies but can be well over 100 knots on the likes of a 777) the rudder hasn't got enough airflow over it to be fully effective in countering the yaw produced by a single engine still running at take-off power.
If you have an engine failure on a 777 post V1 the rudder can compensate for the asymmetric thrust.
Replace "can" by "at high speed, above Vmcg". Below Vmcg the rudder won't be able to compensate anything like enough, certainly in the event of an engine failure at low speed with high asymmetric thrust..( the low speed rudder effectiveness problem again). On a triple and probably all big twins if you set take off power on one engine at say, 30-40 knots, you
are going to go off the side, even with full opposite rudder applied .....
This is why people here are commenting that in the event of you (madly) attempting a single engine ferry on a twin you'd need to feed in the (asymmetric) power slowly and wait until the speed has built up, quite possibly to well over 100 knots,, before applying takeoff power.