If you want diversity, sure. However,I admit that if I had free access to Pinot & Cab, I'd probably go at least a bit crazy.
Are you doing enough Petite Sirah for a carboy, either 3 or 5 gallon? If so, ferment separately and look at blending next summer or fall.
Petite Sirah is supposed to be dark and tannic, so it will dramatically change the PInot Noir. If you want more of a Pinot Noir, go light on the Petite Sirah.
OTOH, if you're not doing enough Petite Sirah for a carboy, you may not have much choice except to do a field blend. Personally, it sounds good either way, although the final result of a field blend will be unpredictable. But it will be interesting.