hrm, the way I figure it two 10 Ω resistors in parallel, is going to give you basically a net resistance of 5 Ω, that's why the voltage went up when you added the second 10 Ω resistor and consequentally so did your amp draw and total heat (but the heat per resistor stayed the same). So four 1 Ω resitors in series is going to give you more current draw than the two 10 Ω resistors in parallel, but the temp of each resistor is going to be less, because it will be split between 4 resistors instead of 2.

Basically the short answer is you can go either way, 2 series 2 parallel 10Ω resistors is going to give you a net resistance of 10 Ω, which will give you the 12.30v with a load and w/o 12.5v

four 1 Ω resistors will give you a net result of 4 Ω, which will give you a little better than 12.65v w/o load & 12.5v, and will be less heat per resistor because it will be split between 4 resistors instead of 2.

You've already got the two 10 Ω resistors though, so it is going to be less expensive to just get another two of those, vs 4 of the 1 Ω resistors, so it really all depends on what voltages you are looking to get, and if you want to buy some extra parts or not =)