diff --git a/haystack/pipelines/base.py b/haystack/pipelines/base.py
index 1fdf506227..538d207500 100644
--- a/haystack/pipelines/base.py
+++ b/haystack/pipelines/base.py
@@ -623,6 +623,12 @@ def run(  # type: ignore
         return node_output
 
     def _combine_node_outputs(self, existing_input: Dict[str, Any], node_output: Dict[str, Any]) -> Dict[str, Any]:
+        """
+        Combines the outputs of two nodes into a single input for a downstream node. This is useful for join nodes.
+
+        :param existing_input: The output of the first node.
+        :param node_output: The output of the second node.
+        """
         additional_input = {}
         # Pass keys that appear in both inputs that have the same values
         shared_items = {
@@ -749,16 +755,8 @@ async def _arun(  # noqa: C901,PLR0912 type: ignore
                                         **existing_input.get("_debug", {}),
                                         **node_output.get("_debug", {}),
                                     }
-                                # Pass keys that appear in both inputs that have the same values
-                                shared_items = {
-                                    k: existing_input[k]
-                                    for k in existing_input
-                                    if k in node_output and existing_input[k] == node_output[k]
-                                }
-                                for key in shared_items:
-                                    if key != "inputs" or key != "params" or key != "_debug":
-                                        updated_input[key] = shared_items[key]
-                                # TODO Auto pass on keys that only appear once
+                                additional_input = self._combine_node_outputs(existing_input, node_output)
+                                updated_input = {**additional_input, **updated_input}
                                 if query and "query" not in updated_input:
                                     updated_input["query"] = query
                                 if file_paths and "file_paths" not in updated_input:
@@ -770,9 +768,9 @@ async def _arun(  # noqa: C901,PLR0912 type: ignore
                                 if meta and "meta" not in updated_input:
                                     updated_input["meta"] = meta
                             else:
-                                # TODO Would need to redo the shared items here to work with more than 2 streams joining
                                 existing_input["inputs"].append(node_output)
-                                updated_input = existing_input
+                                additional_input = self._combine_node_outputs(existing_input, node_output)
+                                updated_input = {**additional_input, **existing_input}
                             queue[n] = updated_input
                         else:
                             queue[n] = node_output