Using 2 old psu's to replace 1

I don't recommend doing this, but it is possible.

It wouldn't create a 500W power supply, but it would allow you to spread the load of your system between the two power supplies. This requires a "PSU bridge" (not exactly sure what they're called) that makes it so that both are activated at the same time.

Why would you need a "bridge"? Why not just hook up various components to the different PSUs? That should work: perhaps GPU on one and everything else on the other.

Everything works better if it's all powered on at exactly the same time. The bridge will ensure that. You could do the same by doing some splicing and such, but that introduces the chance of other things going wrong.

that is a bad idea.