Nah, any replacement would be brought in to put pressure on Tosun or even take his place rather than just make the odd cameo and chip in some goals. Strikers probably more than any other position need kept on their toes so I'd expect any player brought in to be a decent upgrade on Niasse.
Over the past decade, I am not sure we have actually had two strikers of the sort you are describing at the same time, so I would not be so quick too assume it will be happening. And, in case you need reminding, Niasse had the 9th best goals/90 rate in the Prem last season at .65 goals/90 mins, so actually improving on his strike rate through a replacement will be very difficult.
Genuine question, when was the last time we had two strikers scoring 10+ goals in a season?