The Real Time Clock generates an interrupt every 1 second.
It's interrupt priority is 2.

During the same time the A/D Converter unit should generate interrupts every 1us.
It's interrupt priority is 1.
At best that gives you 2 instructions for your A/D ISR

The Interrupt routine of the A/D Converter is only gonna serviced, if the Real Time clock has a lower priority then the ADC.(But then the Real Time Clock wouldn't be serviced any more)
Interrupt priority does not 'remove' interrupts, it just delays them or interrupts them.

But I would like to have the ADC interrupt be serviced during the Real Time clock Interrupt is not active
The result of RTC hving a higher priority.

Erik