You could install the antenna on the front dash (pic in Album). That's how I did it in my RSX and it seems to work perfectly. I tested it in many locations inside and out and the reception is just as good.

Splicing two antennas together sounds interesting... For some reason I fear it won't work, though ... or rather it will but it will just garble the transmission together when it hits the tuner. You'd have to install two tuners as well and splice the tuners together behind the head unit.