Could anyone tell me if it is a good idea to apply for tax and audit positions simultaneously in the same firm (i.e. any of the big 4) or whether it would cause problems.
Most firms tell you to limit your applications to one at a time. Check their terms/FAQ section or call HR to confirm exactly what the company line is at this firm.
Point of note: even if a company say you shouldn’t make multiple applications, you probably still could and get away with it, because most companies use automated application systems. However, if you did this, there is a risk that HR would just disregard one of your applications.
Most firms only want you to submit one application per year. Generally speaking, if you used a different email address, you could apply twice to either the same service line or, as in your case, to differing onces.
What you do is up to you- they do ask you not to, so you should bear that in mind, but equally they might well not notice, particularly if it’s for different service lines.