The baseDir parameter also ca be set multiple directories

browser-sync/browser-sync.d.ts

@@ -65,7 +65,7 @@ declare module "browser-sync" {
         }
         
         interface ServerOptions {
-            baseDir?: string;
+            baseDir?: string | string[];
             directory?: boolean;
             index?: string;
             routes?: {[path: string]: string};
@@ -95,4 +95,4 @@ declare module "browser-sync" {
     }
 
     export = BrowserSync;
-}
+}