Solution for what, exactyly: connecting things in "parallel", or applying signals to things while not powered?
Or both?!

Again, if you would explain what you are actually trying to achieve here, people would be able to make appropriate suggestions...



For connecting things in "parallel", you need some sort of Bus:

The term "bus" is short for "omnibus" which means "by, with, of, or for the use of all" - in other words, a bus is something which is specifically designed to be shared by many things.

Common examples of busses are:

1. Memory Bus: All the memory chips share the bus; one particular chip is selected at a time by its address

2. SPI Bus: Many SPI chips share the bus; one particular chip is selected at a time by its Chip Select (or "Slave Select")

3. I2C Bus Many I2C chips share the bus; one particular chip is selected at a time by its address - which forms part of the I2C communications protocol.



For things which may be powered down, you have two choices:

1. Choose things which are specifically designed for this;

2. Provide some kind of interface "buffer" to isolate the things while they are powered down (this could be as simple as jumper links).