As Joe stated, if you are selling ahome 3-6 months after purchase, FHA requires a second appraisl (it used to be required if you were selling within the year)-paid for by the lender as opposed to the borrower.
Additionally, if-as in your case-there is a significant increase in price, they will require a second appraisal if you are selling within a year from purchase.
If, as JO stated, the second appraisal was ordered by the lender because they were just squirmish about the deal, then they have the right to average the two appraised values, but it is not a guideline issue-rather a comfort decision made by the underwriter.
Please realize that one of the reasons they might order a second appraisa is because they didn't like the first one's comps, etc. in which case they are protecting themselves by ordering a second appraisal and might not look at the first on at all. Files get audited from time to time by FHA (even if the loan is not in default) and the underwriter might be questioned if they used a questionable appraisal.
It can never hurt to send in comps to the appraiser through your realtor, but the appraisers might have seen those comps and not used them for a variety of reasons, in which case they will continue to refuse them.