There are two ADC, let's call them ADC0 and ACD1. You select the first one and do the channels, then the other one.
With interleaving, I mean: If you would let ADC0 sample a channel, and meanwhile go to ADC1 and let that one sample a channel, then return to ADC0 to check if there is data.
If you want to use the interleaving and use millis(), then you can look at the other topic: I2C data is too slow - #11 by Koepel
Your sketch is soooooo slow 
Suppose you send 100 characters of text in each loop() iteration with 9600 baud. That takes about 100ms. Your whole sketch might be slowed down a hundred times, because the Serial.print() functions wait until a new character can be send to the computer.
Use 115200 baud to start with, try if a higher value is possible.
Send less data to the computer, a single space or comma is enough to separate the numbers.
Have you told which Arduino board you use ? I can not find it. Some boards use the USB speed instead of the baudrate. That is even faster than the highest baudrate.