Double-up leads ?

You can run the amp at a 2-ohm stereo load which is basically hooking up two 4-ohm speakers per channel.  I think that's what you meant by "doubling up" but I wasn't quite sure.  However, you can't combine the power from the head unit with the power of the aftermarket amplifier and use it to power a single speaker.  Only one amplification source can power each speaker.

Many of the newer aftermarket head units (particularly ones with MOSFET amp chips) can run 2 ohm stereo loads without overheating.  I think your Pioneer DEH-P3500 is one of them.  And every external aftermarket Class AB 2 and 4 channel amp I've ever seen can run a 2 ohm stereo load as well.

Make sure that there is a good airtight seal around the speakers that you installed in the doors, and that there aren't any holes where air could escape.  Doing this will ensure that you get the most possible bass from the door speakers.  I'd recommend keeping them hooked up to the pioneer's built in amplifier and making sure there's a good seal, before thinking about getting another amp just for those speakers.

Sorry Ethan,

Looks like the top half of my post was cut off.  I am not currently using an amplifier.  I plan on using one for my rear Dynaudio drivers that I own, but have yet to install. 

Im trying to avoid having to buy a new amp, just for my front speakers, and was iquiring as to if I can double-up the F/R leads from my deck into just my front speakers to create more power.  I was hesitant to call this "bridging" because Im not quite sure if it is.

The 6.5s in the door sound fine at low levels, but start to distort at levels they shouldnt.  That is why Im guessing they need more power.  Plus Im hoping that using my 80Hz HPF will help here.

Incidentally, I do have an old Coustic Powerlogic 160u sitting in my closet, I haven;t used in 10 years.  I know it is 35w RMS and can safely run 50w at 2ohms.

A 50w 2ohm signal to my Infinity 6.5 should be OK wouldn't you say?  I guess if the ole amp fails, it won't be that sad.  It isnt worth much...

Oh, and BTW - my deck does have a MOSFET chip.  How would I go about running the two front channels at 2ohms?

Thanks for the help.