Something like this might work for you?...

T foo(Reader<T> reader1, Reader<T> reader2) {
    Reader<T> readerA, Reader<T> readerB;
      // change "getCantCol() == 8" to the verification of a known parameter
    if( reader1.getCantVol() == 8 ) {
       readerA = reader1;
       readerB = reader2;
    }
    else {   
       readerA = reader2;
       readerB = reader1;
    }
    ....
}