src_dir=source1
src_dir=source2
dst_dir=dest1
whatever_thing=thing1
whatever_thing=thing2

src_dir=(source1 source2)
dst_dir=(dest1)
whatever_thing=(thing1 thing2)

while read -r -a line
do
    IFS='= ' read -r -a array <<< "$line"
    "${array[0]}"+="${array[1]}"
done < file.txt