FAN SELECTION

Two identical stages will produce twice the head of one. Flow will remain the same for the same head. Better or worse - who knows?

Apart from that your question is far too vague and the use of capitals is to be discouraged.

2 stage may have a small efficiency advantage because the 2nd stage cancels out the whirl velocity from the first one.

Of course the pressure requirement needs to be high enough to justify the use of 2 stages.

If only a low head is needed, a single stage fan with the appropriate pressure recovery stator will be cheaper and better.

There is a big area of overlap and in most cases the difference is marginal, although the cost will usually favor a single stage.

I can only give a very general and equivocal answer unless you are more specific about your needs.