Add two-clip support for Join Extend mode

SourcePrep gets optional source_clip_2 input — when connected in Join
Extend mode, joins two separate clips instead of splitting one in half.
MergeBack gets optional original_clip_2 to reconstruct the full result
from both originals. Single-clip behavior is unchanged.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

merge_node.py

@@ -106,9 +106,12 @@ Blend methods:
                 "blend_method": (["optical_flow", "alpha", "none"], {"default": "optical_flow", "description": "Blending method at seams."}),
                 "of_preset": (["fast", "balanced", "quality", "max"], {"default": "balanced", "description": "Optical flow quality preset."}),
             },
+            "optional": {
+                "original_clip_2": ("IMAGE", {"description": "Second original clip for Join Extend with two separate clips."}),
+            },
         }
 
-    def merge(self, original_clip, vace_output, vace_pipe, blend_method, of_preset):
+    def merge(self, original_clip, vace_output, vace_pipe, blend_method, of_preset, original_clip_2=None):
         mode = vace_pipe["mode"]
         trim_start = vace_pipe["trim_start"]
         trim_end = vace_pipe["trim_end"]
@@ -120,9 +123,15 @@ Blend methods:
             return (vace_output,)
 
         # Splice modes: reconstruct full video
+        two_clip = vace_pipe.get("two_clip", False)
         V = vace_output.shape[0]
         head = original_clip[:trim_start]
+        if two_clip and original_clip_2 is not None:
+            tail = original_clip_2[trim_end:]
+            right_orig = original_clip_2
+        else:
             tail = original_clip[trim_end:]
+            right_orig = original_clip
         result = torch.cat([head, vace_output, tail], dim=0)
 
         if blend_method == "none" or (left_ctx == 0 and right_ctx == 0):
@@ -142,7 +151,7 @@ Blend methods:
         for j in range(right_ctx):
             alpha = 1.0 - (j + 1) / (right_ctx + 1)
             frame_idx = V - right_ctx + j
-            result[trim_start + frame_idx] = blend_frame(original_clip[trim_end - right_ctx + j], vace_output[frame_idx], alpha)
+            result[trim_start + frame_idx] = blend_frame(right_orig[trim_end - right_ctx + j], vace_output[frame_idx], alpha)
 
         return (result,)
 

nodes.py

@@ -323,7 +323,7 @@ input_left / input_right (0 = use all available):
   Pre Extend:          input_right = leading reference frames to keep
   Middle Extend:       input_left/input_right = frames each side of split
   Edge Extend:         input_left/input_right = start/end edge size (overrides edge_frames)
-  Join Extend:         input_left/input_right = edge context from each half
+  Join Extend:         input_left/input_right = edge context from each half (or each clip if source_clip_2 connected)
   Bidirectional:       input_left = trailing context frames to keep
   Frame Interpolation: pass-through (no trimming)
   Replace/Inpaint:     input_left/input_right = context frames around replace region
@@ -385,6 +385,12 @@ input_left / input_right (0 = use all available):
                 ),
             },
             "optional": {
+                "source_clip_2": (
+                    "IMAGE",
+                    {
+                        "description": "Second clip for Join Extend — join two separate clips instead of splitting one in half.",
+                    },
+                ),
                 "inpaint_mask": (
                     "MASK",
                     {
@@ -401,7 +407,7 @@ input_left / input_right (0 = use all available):
             },
         }
 
-    def prepare(self, source_clip, mode, split_index, input_left, input_right, edge_frames, inpaint_mask=None, keyframe_positions=None):
+    def prepare(self, source_clip, mode, split_index, input_left, input_right, edge_frames, source_clip_2=None, inpaint_mask=None, keyframe_positions=None):
         B, H, W, C = source_clip.shape
         dev = source_clip.device
 
@@ -464,6 +470,10 @@ input_left / input_right (0 = use all available):
             return (output, mode, 0, sym, mask_ph(), kp_out, pipe)
 
         elif mode == "Join Extend":
+            if source_clip_2 is not None:
+                first_half = source_clip
+                second_half = source_clip_2
+            else:
                 half = B // 2
                 first_half = source_clip[:half]
                 second_half = source_clip[half:]
@@ -475,7 +485,14 @@ input_left / input_right (0 = use all available):
             part_2 = first_half[-sym:]
             part_3 = second_half[:sym]
             output = torch.cat([part_2, part_3], dim=0)
-            pipe = {"mode": mode, "trim_start": half - sym, "trim_end": half + sym, "left_ctx": sym, "right_ctx": sym}
+            two_clip = source_clip_2 is not None
+            if two_clip:
+                trim_start = first_half.shape[0] - sym
+                trim_end = sym
+            else:
+                trim_start = half - sym
+                trim_end = half + sym
+            pipe = {"mode": mode, "trim_start": trim_start, "trim_end": trim_end, "left_ctx": sym, "right_ctx": sym, "two_clip": two_clip}
             return (output, mode, 0, sym, mask_ph(), kp_out, pipe)
 
         elif mode == "Bidirectional Extend":