Merge with more than a single column for `left_on` possible?

@eddyxu @changhiskhan I had asked about adding columns in #767 and implementing this got put on hold, but now I have some questions.

Is the left_on and right_on only able to select a single column (it seems so)? We typically have datasets with inference_date and id, where for each day (inference_date there are multiple overlapping ids. We could make a column like date_id and join those two columns together to be able to do the merge. I am just checking if this feature will ever be possible or how best to design this from our end.

Thanks!