Ran a test on PS!
With Merciless
Without Merciless
But unfortunately, the test isn't clear in result, so I ran some Damage Calculations, and it looks like you're right: Both Double Damage and Crit Hit stack up
0 SpA Toxapex Venoshock vs. 0 HP / 0 SpD Abomasnow: 98-116 (30.5 - 36.1%) -- 42.4% chance to 3HKO [Neither Merciless nor Poisoned]
0 SpA Toxapex Venoshock (130 BP) vs. 0 HP / 0 SpD Abomasnow: 194-230 (60.4 - 71.6%) -- guaranteed 2HKO after toxic damage [Poisoned, but no Merciless]
0 SpA Toxapex Venoshock vs. 0 HP / 0 SpD Abomasnow on a critical hit: 146-174 (45.4 - 54.2%) -- 38.7% chance to 2HKO [Crit hit, without Poison]
0 SpA Toxapex Venoshock (130 BP) vs. 0 HP / 0 SpD Abomasnow on a critical hit: 290-344 (90.3 - 107.1%) -- 43.8% chance to OHKO [Crit hit, with Poison, i.e. with Merciless]
