I'm playing a Gloom Pact Hexblade, and I've already taken flail expertise for the sweet at-will prone. In paragon tier, I'm taking Lashing Flail (MC Executioner to qualify as a martial class). My question is could you use your at-will, drop them prone by sacrificing the build in slide with flail expertise, and then slide from Lashing Flail? The way I see it breaking down is Make the attack, sacrifice my slide to knock them prone, then slide one square from the feat. Normally the two slide 1 aspects wouldn't stack, but could I use up one while keeping the other?