BIDS: include physio readings of same type but with different units

Hello neurostars,

I’d like to incorporate, in a BIDS valid way, two respiratory recordings. The two readings have the same sampling frequency and start time, but not the same units.
According to the documentation, for a single respiratory recording, I can specify units for my column as

   "SamplingFrequency": 100.0,
   "StartTime": 0,
    "Columns": ["respiratory"],
   "respiratory": {
       "Units": "m³/s"

But in my case I have two respiratory recordings so the Columns field is

"Columns: ["respiratory", "respiratory"]

How can I add unit fields that would refer specifically to the first respiratory data and the second separately?

I guess I could simply split my data into two single-column .tsv files, each having their own .json file, but I wonder if there a more elegant way of achieving my goals.

Thanks for your input!