Ethernet: two input signals to one pc

It sounds like the two connections should remain separate from each other, as the lab device might require different IP configuration than your local network does (and probably is not supposed to interact with the local network for security).

In this case, none of the above. You want to get a second completely separate Ethernet interface instead. (There are PCI and PCI-E cards for desktops, as well as USB Ethernet adapters for laptops.)

On the other hand, if the lab device is supposed to be part of your LAN network, you would want an Ethernet switch. (Note that most devices nowadays sold as "hubs" are switches, not actual hubs.)

Passive Ethernet splitters aren't suitable for either task. They don't "merge" the signals, only carry two separate signals over one cable that needs to be split back into two ports again.