Improve performance of two batch overlap in some imbalanced cases (#6593)

python/sglang/srt/two_batch_overlap.py

@@ -40,13 +40,21 @@ def compute_split_seq_index(
 
 def _split_array_by_half_sum(arr: Sequence[int]) -> int:
     overall_sum = sum(arr)
-    accumulator, split_index = 0, 0
-    for value in arr[:-1]:
-        accumulator += value
-        split_index += 1
-        if accumulator >= overall_sum // 2:
+    left_sum = 0
+    min_diff = float("inf")
+    best_index = 0
+
+    for i in range(1, len(arr)):
+        left_sum += arr[i - 1]
+        right_sum = overall_sum - left_sum
+        diff = abs(left_sum - right_sum)
+        if diff <= min_diff:
+            min_diff = diff
+            best_index = i
+        else:
             break
-    return split_index
+
+    return best_index
 
 
 def compute_split_token_index(