Much like you can plug more than one item into the home mains supply, you can power more than one item from a battery or other power source.
At a high level, as long as your PSU is capable of supporting the combined load of RPI and a screen, then you should be fine. You need to make sure that both the RPI and screen operate from the same voltage. If they don't, then you need a regulator or some other converter, or a separate PSU/battery.
You can check the maximum current consumption of the RPI by checking the hardware page for it.
