Unfortunately I don't think the current version of QIIME2 demux command is able to demutliplex the "combinatorial barcodes" as in your case (where the same barcode sequence is used in more than one sample), please see:
I believe you have to find a tool to demultiplex the data outside QIIME2, then import them as demultiplexed samples.
Keep up posted how it goes
Luca