diff --git a/deepxde/nn/pytorch/mionet.py b/deepxde/nn/pytorch/mionet.py
index 6001f5ffb..7f723134f 100644
--- a/deepxde/nn/pytorch/mionet.py
+++ b/deepxde/nn/pytorch/mionet.py
@@ -5,8 +5,8 @@
 from .. import activations
 
 
-class MIONetCartesianProd(NN):
-    """MIONet with two input functions for Cartesian product format."""
+class MIONet(NN):
+    """Multiple-input operator network with two input functions."""
 
     def __init__(
         self,
@@ -29,9 +29,9 @@ def __init__(
             self.activation_branch2 = activations.get(activation["branch2"])
             self.activation_trunk = activations.get(activation["trunk"])
         else:
-            self.activation_branch1 = (
-                self.activation_branch2
-            ) = self.activation_trunk = activations.get(activation)
+            self.activation_branch1 = self.activation_branch2 = (
+                self.activation_trunk
+            ) = activations.get(activation)
         if callable(layer_sizes_branch1[1]):
             # User-defined network
             self.branch1 = layer_sizes_branch1[1]
@@ -81,6 +81,66 @@ def __init__(
         self.merge_operation = merge_operation
         self.output_merge_operation = output_merge_operation
 
+    def forward(self, inputs):
+        x_func1 = inputs[0]
+        x_func2 = inputs[1]
+        x_loc = inputs[2]
+        # Branch net to encode the input function
+        y_func1 = self.branch1(x_func1)
+        y_func2 = self.branch2(x_func2)
+        if self.merge_operation == "cat":
+            x_merger = torch.cat((y_func1, y_func2), 1)
+        else:
+            if y_func1.shape[-1] != y_func2.shape[-1]:
+                raise AssertionError(
+                    "Output sizes of branch1 net and branch2 net do not match."
+                )
+            if self.merge_operation == "add":
+                x_merger = y_func1 + y_func2
+            elif self.merge_operation == "mul":
+                x_merger = torch.mul(y_func1, y_func2)
+            else:
+                raise NotImplementedError(
+                    f"{self.merge_operation} operation to be implimented"
+                )
+        # Optional merger net
+        if self.merger is not None:
+            y_func = self.merger(x_merger)
+        else:
+            y_func = x_merger
+        # Trunk net to encode the domain of the output function
+        if self._input_transform is not None:
+            x_loc = self._input_transform(x_loc)
+        y_loc = self.trunk(x_loc)
+        if self.trunk_last_activation:
+            y_loc = self.activation_trunk(y_loc)
+        # Dot product
+        if y_func.shape[-1] != y_loc.shape[-1]:
+            raise AssertionError(
+                "Output sizes of merger net and trunk net do not match."
+            )
+        # output merger net
+        if self.output_merger is None:
+            y = torch.mul(y_func, y_loc)
+            y = torch.sum(y, dim=1, keepdim=True)
+        else:
+            if self.output_merge_operation == "mul":
+                y = torch.mul(y_func, y_loc)
+            elif self.output_merge_operation == "add":
+                y = y_func + y_loc
+            elif self.output_merge_operation == "cat":
+                y = torch.cat((y_func, y_loc), dim=1)
+            y = self.output_merger(y)
+        # Add bias
+        y += self.b
+        if self._output_transform is not None:
+            y = self._output_transform(inputs, y)
+        return y
+
+
+class MIONetCartesianProd(MIONet):
+    """MIONet with two input functions for Cartesian product format."""
+
     def forward(self, inputs):
         x_func1 = inputs[0]
         x_func2 = inputs[1]
@@ -170,9 +230,9 @@ def __init__(
             self.activation_trunk = activations.get(activation["trunk"])
             self.activation_merger = activations.get(activation["merger"])
         else:
-            self.activation_branch1 = (
-                self.activation_branch2
-            ) = self.activation_trunk = activations.get(activation)
+            self.activation_branch1 = self.activation_branch2 = (
+                self.activation_trunk
+            ) = activations.get(activation)
         self.pod_basis = torch.as_tensor(pod_basis, dtype=torch.float32)
         if callable(layer_sizes_branch1[1]):
             # User-defined network