PaulS:
Serial.print() just stuffs the data in a buffer. Interrupts have to happen for that data to actually be shifted out. If the outgoing buffer fills up, Serial.print() blocks until there is room for the data it wants to put in the buffer to fit. That will never happen, because interrupts are disabled while your interrupt service routine is running.
Can my interrupt wait for the other one to happen? This is a general question, since in this case it will have an effect on the performance of the ADC. Do we have any access to that interrupt that controls the flush of buffered data to the serial port?