They are the same. Some people also refer to these as the SYNERGY INDEX. The theory is simple... if the fractional concentrations add up to less then 1, then you have synergy. However, many researchers nowadays believe that anything bellow 0.5 (instead of 1) is synergy. The reason for this change is that two-fold difference in MICs is common and acceptable for any MIC-based assay; so it's taken into account.

thanks