The coalescence process of liquid marbles is vital to their promising roles as reactors or mixers in digital microfluidics. However, the underlying mechanisms and critical conditions of liquid marble coalescence are not well understood. This paper studies the coalescence process of two equally-sized liquid marbles via vertical collision aided by dielectrophoretic handling. A liquid marble was picked up using the dielectrophoretic force and then dropped vertically onto another liquid marble resting on a hydrophobic powder bed. The whole collision process was recorded by a high-speed camera and the recorded images were then analysed to derive the generalised conditions of liquid marble coalescence. By varying the marble volume, impact velocity and offset ratio in the experiments, we concluded that liquid marble coalescence may occur through the coating pore opening mechanism. We quantitatively measured the radius change versus time of the liquid neck formed between two coalescing marbles and estimated the maximum deformation of impacting marbles before rupture in rebound cases. We also qualitatively described the redistribution of coating particles at the impact area during coalescence as well as the consequent ejection of particles. Finally, we summarised the critical conditions for liquid marble coalescence, providing a frame for future applications involving liquid marbles as micromixers and microreactors.