depth region -L for multiple bam

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I try:
sambamba depth region -L $bed $sample1.bam $sample2.bam ... > output.bed

and I expected to get multiple coverage columns that indicate the multiple input bams in one output, but it will merge all bams into one and only get the sum coverage in one column. I do not know whether it is a bug but I think what I expected may be better.
Thanks!