It is relatively unlikely that Reversed Phase (RP) and Aqueous Normal Phase (ANP) HPLC retention contributions would be equal for different amphiphilic compounds because it would require that both mechanisms would be operating at equal efficiency for those particular compounds.  In most cases one mechanism would be more predominant than the other and retention would be different.  However, even in the unlikely event that both were equal this would only occur at one mobile phase composition.  

Therefore, you could switch to a different isocratic composition or a different gradient and that would shift the relative contributions of the two mechanism allowing separation.

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