I see that at the starting date of the EB campaign, Pahlava and Bactria are allies. From reading "Thundering Zeus" (I'm actually at the beginning of the book), It is said that Arsaces was driven out of Bactria by Diodotus and then attacked semi-independent satrap of Seleucid Parthia, where he was more successful. Only Diodotus II made peace with Pahlava and even allied with them against Seleucids.

So, what to believe?