Can I make a dual output USB charger by connecting two USB sockets 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.