By default plurks can be automatically posted on Twitter; but not the other way around. There used to be services like Mechanical Plurk and Feedtweeter but they are in hiatus now. Are there any reliable services to do this?
Ping.fm is dead but Composer.io supports Plurk and many of the other services that Ping.fm was supporting.
There's an API so you could easily automate the whole process even more. The Twitter API is kind of restricted so it's unlikely that Twitter->Composer->Plurk will come from there but you can definitely write some code yourself to do this.