Be cautious: a direct wired splitter is not the safest approach when mixing an older high-trigger-voltage flash with a modern low-voltage flash and camera. The bigger risk is usually the camera body, since modern cameras are generally designed for low trigger voltages, while older flashes can present much higher voltages.

Based on the advice given, the safest choices are:

• use a Safe-Sync/Sync-Safe type adapter on the older flash, or
• isolate one flash with optical or wireless triggering.

Those approaches avoid exposing the camera or the newer flash to voltage from the older unit through shared wiring. Some radio triggers are also designed to be safe with older high-voltage flashes.

So yes, you should be worried about simply hardwiring them together unless you know the splitter/adapter provides proper isolation. If it does not, avoid the direct electrical connection and use voltage isolation or optical/radio triggering instead.