Deep-water megasplay faults may promote or limit earthquake rupture and tsunami genesis. To better understand how megasplay faults affect earthquake rupture and associated tsunami potential, we build on recent modeling efforts based on observations of coseismic ruptures in the Japan Trench forearc and Chile Margin. We model the upper plate as a wedge that is partitioned into a seismic (velocity-weakening) inner wedge and an outer aseismic (velocity-strengthening) wedge, combined with a splay fault rooting from the decollement. We examine the effects of dip and friction along the splay fault and the width of the outer (velocity-strengthening) wedge during earthquake rupture. Our results suggest that along-strike variations in width of the velocity-strengthening outer wedge along the Chile Margin may play a key role in splay fault activity in the rupture segment of the 2010 Maule earthquake. However, our model fit to the published slip distribution for the 2010 Maule earthquake, suggests that megasplay fault activation did not significantly impact earthquake size along the SC Chile Margin. In contrast, our model fit to the slip distribution for the 2011 Tohoku earthquake shows that megasplay fault reactivation may have moderately affected earthquake coseismic rupture. Splay faults can slip coseismically thus contributing to associated tsunamis. However, the presence of a velocity-strengthening outer wedge is the predominant constraint on rupture size and tsunami generation.