Dear Leonard,
My understanding is that you have two sets of equipment at each site: a series of man-operated instruments plus an AWS (automatic instruments).
If this is the case, e.g. at Rotuma Island station, it is not expected that this station will be disseminating two (weather) data streams at the same time, only one data stream of the same kind, such as weather data, is expected from each site, at a certain time, therefore, you just need one WIGOS Station Identifier (WSI) for each site, i.e. for each station. This means that at each site, you may have more than one set of equipment, but they constitute one station only.
You may wish to collect at the site, data from both sets of equipment at the same time, however, at each time there will be only one source of data being disseminated internationally, even if there is a mix of data sources at a certain time; For example, at date-time xyz Rotuma Island station may be sending to GTS the following data: pressure from the manned-instrument, temperature from the AWS, precipitation from the manned-instrument, wind from the AWS, etc.
In conclusion, you don't need two WIGOS station identifiers for Rotuma Island station - this is only one station, so you can use the current WSI (0-20000-0-91650) for any observations coming from this site.
In summary, my recommendation would be to keep one WSI per site, preferably the current ones, unless you/your service have different and strong reasons for not doing so.
Finally, it is important to mention that your country defines a WSIs strategy, basically a set of rules describing how a new station (new site in your country) will be assigned with a WSI, in particular, your country should define how the 3rd and 4th blocks of the WSIs will be used. This is a task that the NFP on WIGOS could coordinate at national level.
I hope this makes sense to you,
Thank you, best regards,
Luis