Easiest way to connect two pins when an output is off?
If i understand the question, you could do it like this.







simulate this circuit &ndash; Schematic created using CircuitLab

Although this seems somewhat redundant to me, as this is essentially how the ATiny works internally. A more elegant solution would seem to be connecting them together directly, like in the following diagram. The use of a discrete transistor is sometimes required, but only if it needs to sink more than the capacity of the controller, and it looks like it's only overpowering a pull-up resistor, piece of cake.







simulate this circuit

Note that this will only work if the 2 circuits are at the same ground potential, which can usually be achieved by connecting the grounds directly together.

You should also take into consideration that the 2 circuits may be running on different logic voltages, so to avoid problems there you should be setting the pin on the ATiny to a hi-z state (in arduino, "input" mode) to avoid mucking with the other circuit.