Variant calling from merged bam file with one vcf output

Question

[askif4]

I have merged two different bam files with same @RG ids (with picard)

When I looked into the bam file however,

I found that the name "test" has changed.

I have matched the IDs for variant calling for one merged bam file.

When I tried merging different bam files and did normal variant calling, the output vcf file had more than one sample columns at the end. (the image below is an example for sample columns)

I want that sample column to be one.

How can I make merged bam files to be considered as one big bam file?