The platformer tutorial has a good solution I've been using
On "A" key down
> Set mirrored
> Simulate Platform Left
On "D" key down
> set not mirrored
> simulate platform right
However, if you have separate Left and Right animations, you can replace the Mirrored function with a boolean if they are going left or not, and in your animations differentiate it there.
Platform Moving
>Set animation running
Platform moving
>if BOOLEAN
>>Set animation left running
>if NOT BOOLEAN
>>Set animation right running
Could also add an "platform on ground" to the moving checks as well if you want to make sure.
My animations are pretty smooth, that's not an issue I have.
The issue I have is sometimes the walk-left animation is playing (likely due to a previous "walk left" command) when the character is moving right.