It is a really difficult task finding a useful second striker. The likes of the top 6, and us with Lukaku, play their main striker as much as possible. The second striker only gets a look in when he comes on when to mix the game up or if there's an injury, but even then managers are plugging the gap with wingers.
So there is the problem. You can't sign a top class forward to play a secondary role because he'll be unsettled. Conversely you are going to want one who is able to hold his own, get goals, create and be a genuine solid back up option.
This is why I do have my reservations about Berahino. Seems like he wouldn't be happy with a stint on the bench even for a bigger club.
Best use of forward options I've seen this season is Arsenal's. They have a winger who can play striker (Sanchez) an out and out striker (Giroud) and a good back up (Perez) who has linked well with Giroud. I wouldn't mind seeing us use the same approach - A good back up forward who can support and a winger who can play forward.