In Duplicati, deduplication does not work across multiple backup jobs, so uploading backups to the same folder will not help to save storage.
Generally, it is not recommended to upload multiple backups to the same location. Backup job 1 will detect unrecognized filed created by backup job 2 and vice versa.
If for whatever reason you still want to put multiple backup jobs in the same folder, give the uploaded volumes of all backup jobs a unique prefix by supplying the --prefix option to every backup job.