No official ruling, unfortunately, but they have argued it on the Paizo forums about it. Mostly, the general consensus is, while Double Slice lets you take the full strength bonus on the damage with offhand weapons, Power Attack isn't a strength bonus based damage bonus, it's a flat damage bonus, and so on offhand is always half (at least in this case. Monks would be different with Flurry of Blows unarmed, obviously.)
It seems the common consensus is, the half penalty on the offhand is to make it more inline if someone was using 1 2-handed weapon at 1-1/2 the strength bonus, and not make it more powerful than that.
The thread on Paizo's forum is
here, if you wanted to look at it. It's old, but still relevant.
Thanks for the response!