There are many major problems standing in the way of what you want.
You could run 20 instances of weewxd, each inserting into the same database table, each with its own station ID. The problem is the daily summaries. While you may have only one main archive table, you would need 20 different sets of daily summaries, one for each station. Alternatively, have a single set of daily summaries, keyed by the station ID. Either way, it's a big project.
The second problem is that WeeWX reporting has no way of specifying which station(s) you wish to include in the reports. So, this would have to be added. Also a big project.
Personally, if I was faced with this problem, I would run 20 instances of weewxd, inserting into 20 different MySQL databases. Then use something like Grafana to display them, possibly by using SQL UNIONs.
I sympathize, but you'll have to figure this out yourself. It's just not a priority for the development team.
-tk