Are the 2 Vss pins on a 6502 connected internally? Or do they drive separate parts of the chip? I'm designing a circuit on perfboard and I only have space to connect one of them to GND, will that cause a problem? What about then using the other Vss pin to source a Vss on, say, an EEPROM chip? Is it ok to treat them like this i.e. as though there's a regular wire running through the chip connecting them?
You have to connect them both. Yes, they're connected internally, but that doesn't mean that you can get away with connecting only one of them externally.
And you definitely cannot use them as a pass-through to some other chip.
Why would you have any restrictions on how many pins you can connect on perfboard anyway? It's just more wires, right?