Comb electron beams are fs and high brightness electron bunches used to drive plasma accelerating modules. An ultra-relativistic beam of charged particles (the driver) through a plasma generates a wake of charge density useful for accelerate a witness beam (Particle driven WakeField Acceleration - PWFA). The witness dynamics control is fundamental to achieve the optimum transverse and longitudinal matching at the plasma entrance. Beam dynamics simulations were performed under velocity bunching conditions to observe how the witness (about 30pC) bunch dynamics changes due to the driver (about 200pC). A pulse shaping parametric scan has been performed starting from a well-defined initial condition, i.e. EUPRAXIA @SPARC_LAB working point. This working point is a 30-pC witness, 290 fs long gaussian beam with 0.175 mm transverse dimension and a 200-pC driver, 230 fs long gaussian beam with 0.35 mm transverse dimension. The 100 MeV Linac has been optimized to achieve optimal emittance, spot size, energy, energy spread and longitudinal distance between bunches. SPARC_LAB machine layout has been used for this analysis with ASTRA simulation tool.