I am looking to use Apache Superset and am considering if it will allow for me to connect to both POSTGres and MSSQL at the same time. 
There might be instances of creating queries from both databases however, i cant figure out based on the API documentation if this can be done. 
non-dev here.
You can indeed connect to both databases. Superset lets you connect many databases (and types thereof!) by adding various connection strings.
However, you cannot do JOIN queries between the two databases. Superset takes your query and ships it down to the database to do the work. In order to join two databases in a query, Superset would have to basically do an ETL job pulling in both databases and doing the query there. That's not how it's built. 
But again, you CAN have multiple data sources and have queries/charts that call out to each of them, all rolled into one dashboard.
Multiple database access on Apache Superset?