Here's the scenario. 16ch snake going into a 16ch mixer. Mixer has 16 individual outputs going into 8ch Focusrite  18/20 audio interface which is connected to a 8ch Presonus DigiMax audio interface via ADAT cable. In order to activate condenser mics that require phantom power i need to activate it from the mixer.
 
Question - will having the audio interface phantom power on affect the single? or will it cause issues?

Basically I would have two phantom powers running at the same time

Not sure I understand your configuration, but two phantom power sources can not be connected together...very bad things can happen, including releasing the magic smoke!
 
If you could try to explain your requirements again I might have a solution.

Hey Bill - so lets say I have a condenser vocal mic plugged into ch1 on the snake in the live room. That's plugged into ch1 on the mixer. I engage the phantom power on ch1 on the mixer as well as ch1 on the audio interface. GOOD or NOT GOOD tis the question. I think it's not good

I think the audio interface phantom power is not required. The mixer already done this.
 
Mixer to audio interface is via XLR ?

mtgonzalez
Hey Bill - so lets say I have a condenser vocal mic plugged into ch1 on the snake in the live room. That's plugged into ch1 on the mixer. I engage the phantom power on ch1 on the mixer as well as ch1 on the audio interface. GOOD or NOT GOOD tis the question. I think it's not good

hate to be dense, but what is plugged into the audio interface, the microphone or the direct out from the mixer or a mixer output? Sounds like you are plugging the mixer OUTPUTS into the audio interface.
 
If you are plugging the microphone (through the snake) into both the mixer and the audio interface that isn't going to work very well at all, even without the phantom power question.
 
Assuming you are plugging the mixer outputs into the interface inputs there is no need to turn on phantom power in the interface. It probably won't hurt anything, but if the mixer outputs are poorly designed you could damage the mixer and the interface. The microphone would be safe.

Am I making sense?

Only turn on phantom power on channels that are DIRECTLY connected to devices that need it, ie. condenser mics and some DI boxes.  All other phantom power should be turned OFF.

mtgonzalez
Here's the scenario. 16ch snake going into a 16ch mixer. Mixer has 16 individual outputs going into 8ch Focusrite  18/20 audio interface which is connected to a 8ch Presonus DigiMax audio interface via ADAT cable. In order to activate condenser mics that require phantom power i need to activate it from the mixer.
 
Question - will having the audio interface phantom power on affect the single? or will it cause issues?

It seems as if there is no need for your audio interface to have the phantom power running.
 
It seems as if the mixer is the only thing connected to the mics.
 
It seems as if 8 analog channels of the mixer output are going straight to the Focusrite I/O interface.
 
It seems as if another 8 channels of the mixer are routed: mixer analog output>Presonus DigiMax analog in to digital output via ADAT>ADAT to Focusrite I/O interface.
 
It seems as if phantom power on the I/O interface will be "seen" by the mixer at its analog outputs.
 
 
If all those interpretations/assumptions are applicable then the following can be concluded:
 
There is no phantom power on the ADAT bus so the Presonus DigiMax is safe.
 
Will the phantom power on the Focusrite 18/20 hurt the mixer outputs? Maybe. Turn off the Focusrite's phantom power option.
 
Will the phantom power on the Presonus DigiMax hurt the mixer outputs? Maybe. Turn off the Presonus DigiMax's phantom power option.
 
 
 
 
 
 
A more practical concern is the need to determine if the "mixer" has enough phantom "power" to supply your mics through a snake.
 
How many mics?
How much current does each mic want?
How much current can the mixer supply?
How long is the snake?
What guage is the wiring in the snake?

^ what he said

Yup. What he ^^ said.
 
The mixer inputs and outputs are completely separate and isolated circuits. Assuming the snake is balanced, having phantom power on at the interface would have no effect. If the snake is unbalanced, then that could potentially be very bad for both the interface's phantom power supply and the mixer's output amplifiers.