This just came up in my group. It looks like RAW, you can use double slice to throw weapons that qualify as melee... but I feel like the intent of double slice is to only work for melee strikes. If the type of Strike was defined like "Make one melee Strike" then that would fix it (assuming it's meant to not work with thrown weapons.)
Thrown You can throw this weapon as a ranged attack. A thrownweapon adds your Strength modifier to damage just like a
melee weapon does. When this trait appears on a melee
weapon, it also includes the range increment in feet. Ranged
weapons with this trait use the range increment specified in
the weapon’s range entry.
So, I feel there's a clear distinction here made by Paizo specifically, and not some error:There are both melkee AND ranged thrown weapons, and not by a "mistake" or something.
So, I assume that things that work with melee weapons (and not melee attacks) like double slice are supossed to work with melee thrown weapons.
Keep in mind that this works in both ways though: if ther appears something like a Distance property rune sometime in the future, you wouldn't be able to etch it on a melee thrown weapon, but you would on a ranged thrown weapon.