The 2005s still use dual-filament H4 bulbs, right? I think it was 2006 when Foresters switched to separate bulbs. Lots of owners with H4s have encountered problems with melted sockets, even just from normal usage with stock bulbs. So for starters, you'd definitely have to go with the highest temp sockets you can find, or perhaps even just 1/4" crimp terminals with no plastic holding them together.
How about the bulbs themselves? Do you know if they're rated for use with both beams at the same time? If you can't find authoritative info about this, you could set up a bench test. Wear safety glasses!
Then there's the matter of the flimsiness of the stock wires. We lose over two volts already in the stock circuitry.
I think you would be in for a totally new set of heavy gauge wires, relays, and sockets. Typical complicators in driving relays from the stock wiring are interactions with DRLs and with the fog light relay. If you don't care about DRLs you can disconnect them and drive one relay from the stock low beam circuitry. It's a bit harder to drive a relay from the high beam circuitry, as Subaru uses the path through the high beam filament in conjunction with the fog light relay. You can get around this by paralleling your relay with an appropriate resistor to bypass enough current to drive the fog light relay, or you can ditch the stock fog light wiring completely, and set up your own there as well.
Connect one relay to respond to the low beam condition and drive the low beams. Connect two to respond to the high beam condition and drive both beams, one from each.
Exclusive of the simultaneous high/low beam thing, I plan such a wholesale replacement (augmentation?) of the stock circuitry when the warm weather arrives.
