The reason for having two MOSFETs in parallel in the Typical Application Circuit in the datasheet, or the Evaluation Board of this IC the EVAL-LT8277-AZ, is most commonly done to meet the heat dissipation requirements of the MOSFET packages themselves.
Although the efficiency of this board is very good, there are still some expected losses in the MOSFETs of any power converter. Having two MOSFETs in parallel allows the heat to be dissipated over both, so the temperature rise in the individual MOSFET package itself is halved as opposed to just having one MOSFET per phase.
If we look at the MOSFETs used in the EVAL-LT8277-AZ board, their thermal resistance is 41°C/W. A 1W power loss in one of these MOSFETs would result in an overall case temperature of 66°C at room temperature. If we reduced the circuit to one MOSFET per phase, then the overall case temperature per MOSFET would be 107°C at room temperature, which does not leave much headroom if the board were to be mounted in an enclosure etc.