I was the one that brought up Philip being Kivat with an actor.
He's a character in the show, and will definitely be integral to the plot, but until we see if he henshins before Shotaro, it seems like he's providing power to Shotaro. He's the widget of power. Like Kivat was for Wataru.
Go back and watch the first three episodes. Most of the talking while Double is out is done by Shotaro. Double's grunts and such... all in Shotaro's voice. Philip does occasionally talk (with the weird eye effect in episode 1 and 2), and he definitely can take control of his side of Double, but Shotaro is for all intents and purposes, the primary.
Other examples include MegaMan EXE Axess/NT Warrior. Mega's got the powers, but Lan is mostly the one in control. Mega can talk and converse, but he's little more than the armor Lan is wearing.
Hence, Kivat, with an actor.
This could change, as it's possible that Philip can initiate the henshin. But currently there's only one physical DoubleDriver, and Shotaro's got it.