The X perhaps can't.
The newer ones can.
iPhone XS, iPhone XS Max Supports 10W Fast Wireless Charging, Here Are The Best Qi Chargers You Can Get Today
Anyway as I say the most power I've seen anyone measure being able to reliabily pull from front USB ports is 2.5 amps. Which at 5 volts for USB means 12.5 watts.
Meaning 2 newer iphones would be charging at a little over half their max charging speed using 1 USB port.... (and indeed, even the older iphone X if you were charging 2 of them wouldn't quite hit max speed that would require 15 watts)
Using both ports would allow max charging on any of the above, but without a lot left over for any other devices you might be running off a splitter (not sure how the charger handles this)
This is probably why Taptes themselves suggest using a power bank as a buffer if you want max charging speed.
