Have the flags explained
samtools flags 2304 0x900 2304 SECONDARY,SUPPLEMENTARY
-f INT only include reads with all of the FLAGs in INT present 
can an alignment be both secondary and supplementary at the same time? These are mutually exclusive terms.
If you wanted to include only the non-primary alignments and that you would need to do it in two steps.
First select the secondary, then the supplementary.