Hi @Chenyu,
We don't have a baked-in way to demultiplex these (they kind of are demultiplexed in as much as the barcodes are identified already).
But there's a pretty recent tool: PRONAME: Enhancing Taxonomic Accuracy with Nanopore Long-Read Metabarcoding and QIIME2 which looks to handle the upstream processing and generates Q2 results somewhere in the middle of that process. So I might give that whirl and see if it helps.
cc @BDubois