  Fish schools are capable of simultaneous linear acceleration. To reveal the
underlying hydrodynamic mechanism, we numerically investigate how Reynolds
number $ Re = 1000 - 2000 $, Strouhal number $ St = 0.2 - 0.7 $ and wavelength
$ \lambda = 0.5 - 2 $ affect the mean net thrust and net propulsive efficiency
of two side-by-side hydrofoils undulating in anti-phase. In total, $ 550 $
cases are simulated using immersed boundary method. The thrust increases
significantly with wavelength and Strouhal number, yet only slightly with the
Reynolds number. We apply a symbolic regression algorithm to formulate this
relationship. Furthermore, we find that mirror-symmetric schooling can achieve
a \textit{net} thrust more than ten times that of a single swimmer, especially
at low Reynolds numbers. The highest efficiency is obtained at $ St = 0.5 $ and
$ \lambda = 1.2 $, where $ St $ is consistent with that observed in the
linear-accelerating natural swimmers, \eg Crevalle jack. Six distinct flow
structures are identified. The highest thrust corresponds to an asymmetric flow
pattern, whereas the highest efficiency occurs when the flow is symmetric with
converging vortex streets.
