(deep breath)
Ethernet "splitters" are of limited utility even when using Ethernet. When used in pairs (one at either end) they allow you to use a single 8-conductor cable to carry two 4-conductor Fast Ethernet links. They do not function like a 2-port switch or hub.
To plug more than one device into a DH485 port on an SLC-5/0x controller, you properly need an isolator or two. Ideally, two 1747-AIC isolators, or a 1761-NET-AIC isolator/RS-232 converter.
"What would happen if I tried anyway" ?
Neither device would connect to the SLC-5/03 properly.
The DH485 port on the SLC-5/03 controller pinout is:
1 Data B
2 Data A
3 +24V DC
4 Signal Ground
5 Tx-Enable
6 Earth Ground (Shield)
7 Signal Ground
8 +24V DC
An Ethernet Splitter is generally:
Output Pin 1 - Interface 2, Pin 1
Output Pin 2 - Interface 2, Pin 2
Output Pin 3 - Interface 2, Pin 3
Output Pin 4 - Interface 1, Pin 2
Output Pin 5 - Interface 1, Pin 1
Output Pin 6 - Interface 1, Pin 6
Output Pin 7 - Interface 2, Pin 3
Output Pin 8 - Interface 1, Pin 6
A device plugged into interface port 2 of the splitter would get Pins 1 and 2 (Data A and B) connected correctly, but not Pin 4 or Pin 7 (Signal Ground), so it couldn't connect. It would get Pin 3 at +24V, which wouldn't hurt it because it expected that voltage and because it's not connected to Signal Ground.
A device plugged into interface port 1 would get +24V (pin 8) onto what it expected to be Signal Ground (pin 6), and would probably crowbar the SLC-5/03 power supply or smoke the port.