The library does not allow for multiple receivers. But receivers can be connected to the same input pin. That works as long as only one remote is sending at the same time. Otherwise you should use one Arduino for each IR receiver.
If you are using the latest version of the IRremote library (>3.0) that is not correct. See the IRremote library GitHub page for the current usage and how to fix older code to work with the newer version of the library.
"for example" is not very helpful with module internals. But this module, as any other IR receiver I've seen, have an o.c. output with a pullup resistor. Thus at least 2 receiver outputs can be directly connected as "wired AND".
You can calculate the safe amount from the resulting resistance, max. output current and supply voltage. Above module has a 33k pullup that draws 0.15mA at 5V. The guaranteed (LOW) output current is 5mA so that up to 33 receivers can be connected to the same input pin.
Thanks. That is the sensor I am using.
I have altered the PCB design to use the same pin for both IR inputs. I will breadboard that later and ensure it works OK.
I have utilised the spare input pin I now have to detect when the 3.5mm external IR jack is inserted.