Accessing Snakemake Config Samples - bioinformatics

Question

I have a rule that needs to take 2 samples and combine them.
This is how my samples look like in my config file:
samples:
group1:
sra1:
sample: "SRR14724462"
cell_line: "NA24385"
exome_bedfile: "/bedfiles/truseq.sorted.bed"
sra2:
sample: "SRR14724472"
cell_line: "NA24385"
exome_bedfile: "/bedfiles/idt.sorted.bed"
group2:
sra1:
sample: "SRR14724463"
cell_line: "NA12878"
exome_bedfile: "/bedfiles/truseq.sorted.bed"
sra2:
sample: "SRR14724473"
cell_line: "NA12878"
exome_bedfile: "/bedfiles/idt.sorted.bed"
Essentially I want to combine group1 sra1 together, and group2 sra2 together, into these combinations:
SRR14724462 and SRR14724463
SRR14724472 and SRR14724473
This is my rule and rule all:
rule combine:
output:
r1 = TRIMMED_DIR + "/{sample1}_{sample2}_R1.fastq",
r2 = TRIMMED_DIR + "/{sample1}_{sample2}_R2.fastq"
params:
trimmed_dir = TRIMMED_DIR,
a = "{sample1}",
b = "{sample2}"
shell:
cd {params.trimmed_dir}
/combine.sh {params.a}_R1_trimmed.fastq {params.a}_R2_trimmed.fastq {params.b}_R1_trimmed.fastq {params.b}_R2_trimmed.fastq
rule all:
expand(TRIMMED_DIR + "/{sample1}_{sample2}_R1.fastq", sample1=list_a, sample2=list_b),
expand(TRIMMED_DIR + "/{sample1}_{sample2}_R2.fastq", sample1=list_a, sample2=list_b)
This works EXCEPT it does these combinations:
SRR14724462 and SRR14724463
SRR14724462 and SRR14724473
SRR14724472 and SRR14724463
SRR14724472 and SRR14724473
I only want these combinations:
SRR14724462 and SRR14724463
SRR14724472 and SRR14724473
Note: Not shown is how i got list_a and list_b, but essentially they are:
list_a = ['SRR14724462', 'SRR14724472']
list_b = ['SRR14724463', 'SRR14724473']