The problem with the Poacher/Target Man combo is that both strikers will then lack the ability to create chances for each other, leaving that up to the rest of the team.
Granted, a creative target man would fix that problem, but those are rare and expensive. A fast, creative striker would function well as a poacher, but the TC role would waste his talents.
A typical poacher and a typical target man requires the midfield to be creative, good passers. Finding that kind is not difficult, but the defensive balance of the whole tactic could be missing unless you find creative passers with good physique, team work and work rate. But those are rare too.