Cindy's Answer
Walking two dogs together can be tricky unless both dogs understand loose leash walking individually.
I think my advice would be to work on the GSDs loose leash walking so it's really rock solid and then have a second person eventually walk your older dog parallel to you and your GSD. Your young dog needs to understand leash pressure so she doesn't pull in any situation.
At 5 ½ months she's old enough to start expecting a bit more from her but walking with another dog is a pretty advanced skill so I'd break down the expectations into incremental steps. We have a good course on loose leash walking that I've linked below.
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