you may need to adjust some settings in "system preferences -> displays -> arrangement"
I think it defaults to everything being it's own desktop, so you could drag a window from one screen to another. But you can change that so it's displaying the same on all screens.
it might be easier, and provide a better result if you split the HDMI signal after it leaves your computer.
airplay can be a bit finicky at times, and can be quickly impacted by wi-fi issues.
it depends on the nature of the event, a slide show among friends like a birthday dinner or something, airplay might be fine, but in any kind of professional setting i wouldn't use it.
if the monitors are close, you can get a regular splitter. and some short HDMI cables.
if they're further apart, you can get longer HDMI cables with the splitter.
or there are HDMI over cat-5 devices (it uses regular ethernet cable, but you can't run them through a regular switch)
like this
https://www.monoprice.com/product?p_id=8158
**about 50 feet is the longest a standard HDMI cable will go, they make longer, but they'll have some kind of extra electronics to boost them, and prices tend to rise quickly.
check out monoprice.com for splitters and long HDMI cables.
they've got good quality stuff and decent prices.