ASME recommends using multiple tools at the same time. The reason is simple: Simultorc is more accurate and closes flanges the first time right, in just a fraction of the time. HYTORC tools pay for themselves, since bolting time is reduced drastically.
ASME PCC-1-2013 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) The use of the Simultorc™ type of bolting system is recognized by ASME as the preferred method for flanged joint assembly.
ASME PCC-1-2013 (page 33) states: “The simultaneous use of multiple tools spaced evenly around a flange has been shown to give equal or even superior tightening parity, and parallel closure, in less time than using a single tool in a cross-pattern.”