I bought an old wall plug that outputs 5VDC and maximum 2.5 A and thought I'd make a USB charger. Considering the amperage I figured I might as well put two outputs, but now I'm not sure how to connect them. Will it work if I just wire the + and - connectors of the USB ports in parallel?
A parallel connection should be fine. A serial connection would try to to split the voltage between the two devices, and thus wouldn't charge the devices at 5V.
Keep in mind that with some of the wall plugs there's going to be a set of resistors connecting to the D+/D- lines of the output USB-A. This is so the phone "knows" how much current it can draw from the charger.
Here's a reference as to what I mean by the D+/D- lines. I should mention that the link is specific to one brand, but this is common practice for most others as well.