Plugging in the N5 with a wired and wireless charger at the same time would NOT make your Nexus 5 charge faster but it could cause it to overheat that would damage the N5. That coil inside the N5 generates extra heat as a by product of converting magnetic waves into electric.
You wired charger that comes with the N5 is 5 volts @ 1.2 Amps. Since both the coil in the phone and the wired charger connect to the same battery all you are doing is adding 5v to 5v from two parallel sources, that when combined would still be only 5v at the battery. It would increasing the amperage by using two different parallel charging devices that both connect in parallel to the same battery but the N5 would only draw the Amps it needs during charging.
The amperage listed on your charger power supply needs to match or exceed the amperage required by your device. That is why you can use either a 1.2A or 2.1A charger as long as both are at 5 volts. The Nexus 5 will only draw the amperage it needs when charging and there are circuits inside the N5 to limit the amperage it draws to keep the N5 from overheating during charging.
If the connection was in series like multiple batterers in a flashlight (stacked positive to negative positive to negative) that would damage the N5 since it couldn't handle 10 volts for long without damaging the N5's circuits.
This reminds me when I was in college working on an electrical engineering degree. The dorm security had a intercom that could be used remotely to listen into your rooms when the speaker turned into a microphone. The dorm guards busted up my poker game one time and I was put on report. Then we wired 240 VAC @ 30 Amps to the intercom wires in the hall way ceiling and the next time they used it for survalance the guard had to use a fire extinguisher to put out the fire when his Intercom exploded and burned up. They never tried that trick again since none of the security wanted to be near a intercom after that.
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