diff --git a/script/split_components_for_ci.py b/script/split_components_for_ci.py
index 26907936c1..663f2c6d5d 100755
--- a/script/split_components_for_ci.py
+++ b/script/split_components_for_ci.py
@@ -262,8 +262,9 @@ def balance_batches(items: list[_BatchItem], target_seconds: int) -> list[list[s
     """Spread items over enough runners to stay near target_seconds each.
 
     The runner count comes from the total estimate with every grouped build
-    counted once. Heaviest items go first, each to the runner that ends up
-    lightest, so grouped components follow the builds they can join.
+    counted once. Heaviest items go first, each to the runner it adds the
+    fewest seconds to among those ending near the lightest, so grouped
+    components follow the builds they can join.
     """
     if not items:
         return []
@@ -274,10 +275,23 @@ def balance_batches(items: list[_BatchItem], target_seconds: int) -> list[list[s
     total = whole.seconds
     count = min(len(items), max(1, math.ceil(total / target_seconds)))
     batches = [_Batch() for _ in range(count)]
+    # Among the runners ending within slack of the lightest, the one the item
+    # adds the fewest seconds to wins, so a component lands with the bus
+    # group partners it shares builds with; placed away from them it builds
+    # alone on every platform they only share there.
+    slack = target_seconds // 4
     for item in sorted(items, key=lambda i: (-i.standalone_seconds(), i.component)):
-        min(batches, key=lambda b: (b.seconds + b.added_seconds(item), b.seconds)).add(
-            item
+        costs = [(batch, batch.added_seconds(item)) for batch in batches]
+        ceiling = min(batch.seconds + added for batch, added in costs) + slack
+        batch, _ = min(
+            (
+                (batch, added)
+                for batch, added in costs
+                if batch.seconds + added <= ceiling
+            ),
+            key=lambda cost: (cost[1], cost[0].seconds + cost[1]),
         )
+        batch.add(item)
     return [batch.components for batch in batches if batch.components]
 
 
diff --git a/tests/script/test_split_components_for_ci.py b/tests/script/test_split_components_for_ci.py
index 4da644d679..718d754f01 100644
--- a/tests/script/test_split_components_for_ci.py
+++ b/tests/script/test_split_components_for_ci.py
@@ -249,6 +249,24 @@ def test_make_item_reads_tests_dir(tmp_path: Path) -> None:
     assert item.own_seconds == 55
 
 
+def test_balance_batches_keeps_partners_together() -> None:
+    """Two components sharing a build land on one runner even when the
+    lightest runner is the other one."""
+    share = {
+        ("i2c", "esp32-idf"): split_components_for_ci._GroupedShare(55, frozenset()),
+        ("i2c", "esp8266-ard"): split_components_for_ci._GroupedShare(30, frozenset()),
+    }
+    items = [
+        split_components_for_ci._BatchItem("partner_a", 0, dict(share)),
+        split_components_for_ci._BatchItem("partner_b", 0, dict(share)),
+        split_components_for_ci._BatchItem("alone", 80),
+    ]
+
+    batches = split_components_for_ci.balance_batches(items, target_seconds=120)
+
+    assert sorted(batches) == [["alone"], ["partner_a", "partner_b"]]
+
+
 def test_balance_batches_empty() -> None:
     """No components means no runners."""
     assert split_components_for_ci.balance_batches([], 600) == []