Dear Erna,
Considering that the both stations have been in OSCAR/Surface for a long time and have different owners, it seems more beneficial to keep them separate as is. On the other hand, it makes sense to merge them because the synoptic observation is collocated with the upper air observation.
Again, it's up to you to decide how you would like to assign WSIs for stations in your country. We recommend you to follow your National WSI Schema or develop one, if you don't already have it. References and examples of WSI National Schema from other Members are available on the WIGOS webpage, https://community.wmo.int/en/activity-areas/WIGOS/implementation-WIGOS
Thank you, kind regards,
The WIGOS Team