diff --git a/src/openeo_aggregator/backend.py b/src/openeo_aggregator/backend.py
index d9ebb2bb..d815c074 100644
--- a/src/openeo_aggregator/backend.py
+++ b/src/openeo_aggregator/backend.py
@@ -107,17 +107,15 @@ def _get_process_registry(self) -> ProcessRegistry:
                 # TODO: fail instead of warn?
                 _log.warning(f"Failed to get processes from {con.id}", exc_info=True)
 
-        # TODO: not only check process name, but also parameters and return type?
-        # TODO: return union of processes instead of intersection?
-        intersection = None
+        # TODO #4: combined set of processes: union, intersection or something else?
+        # TODO #4: not only check process name, but also parameters and return type?
+        combined_processes = {}
         for bid, backend_processes in processes_per_backend.items():
-            if intersection is None:
-                intersection = backend_processes
-            else:
-                intersection = {k: v for (k, v) in intersection.items() if k in backend_processes}
+            # Combine by taking union (with higher preference for earlier backends)
+            combined_processes = {**backend_processes, **combined_processes}
 
         process_registry = ProcessRegistry()
-        for pid, spec in intersection.items():
+        for pid, spec in combined_processes.items():
             process_registry.add_spec(spec=spec)
 
         return process_registry
diff --git a/tests/test_backend.py b/tests/test_backend.py
index 76f77ef3..f5af6bd0 100644
--- a/tests/test_backend.py
+++ b/tests/test_backend.py
@@ -141,6 +141,29 @@ def test_get_process_registry(self, multi_backend_connection, backend1, backend2
         catalog = AggregatorCollectionCatalog(backends=multi_backend_connection)
         processing = AggregatorProcessing(backends=multi_backend_connection, catalog=catalog)
         registry = processing.get_process_registry(api_version="1.0.0")
-        assert registry.get_specs() == [
+        assert sorted(registry.get_specs(), key=lambda p: p["id"]) == [
+            {"id": "add", "parameters": [{"name": "x"}, {"name": "y"}]},
             {"id": "mean", "parameters": [{"name": "data"}]},
+            {"id": "multiply", "parameters": [{"name": "x"}, {"name": "y"}]},
+        ]
+
+    def test_get_process_registry_parameter_differences(
+            self, multi_backend_connection, backend1, backend2,
+            requests_mock
+    ):
+        requests_mock.get(backend1 + "/processes", json={"processes": [
+            {"id": "add", "parameters": [{"name": "x"}, {"name": "y"}]},
+            {"id": "mean", "parameters": [{"name": "array"}]},
+        ]})
+        requests_mock.get(backend2 + "/processes", json={"processes": [
+            {"id": "multiply", "parameters": [{"name": "x"}, {"name": "y"}]},
+            {"id": "mean", "parameters": [{"name": "values"}]},
+        ]})
+        catalog = AggregatorCollectionCatalog(backends=multi_backend_connection)
+        processing = AggregatorProcessing(backends=multi_backend_connection, catalog=catalog)
+        registry = processing.get_process_registry(api_version="1.0.0")
+        assert sorted(registry.get_specs(), key=lambda p: p["id"]) == [
+            {"id": "add", "parameters": [{"name": "x"}, {"name": "y"}]},
+            {"id": "mean", "parameters": [{"name": "array"}]},
+            {"id": "multiply", "parameters": [{"name": "x"}, {"name": "y"}]},
         ]
diff --git a/tests/test_views.py b/tests/test_views.py
index f1de997f..2027a82b 100644
--- a/tests/test_views.py
+++ b/tests/test_views.py
@@ -97,7 +97,11 @@ def test_processes_basic(self, api100, requests_mock, backend1, backend2):
         ]})
         res = api100.get("/processes").assert_status_code(200).json
         assert res == {
-            "processes": [{"id": "mean", "parameters": [{"name": "data"}]}],
+            "processes": [
+                {"id": "multiply", "parameters": [{"name": "x"}, {"name": "y"}]},
+                {"id": "mean", "parameters": [{"name": "data"}]},
+                {"id": "add", "parameters": [{"name": "x"}, {"name": "y"}]},
+            ],
             "links": [],
         }