Add a default chosen matrix transform in the default keymap, alongside a commented out version for the alternate layout.

app/boards/shields/reviung34/README.md

@@ -4,7 +4,9 @@ REVIUNG34 is a 33-34 key unibody split keyboard by [gtips](https://github.com/gt
 
 By default, the 2x1u layout is used. To use to the 1x2u layout, add the following to your keymap:
 ```
+/ {
     chosen {
         zmk,matrix_transform = &single_2u_transform;
     };
+};
 ```

app/boards/shields/reviung34/reviung34.keymap

@@ -9,6 +9,17 @@
 #include <dt-bindings/zmk/keys.h>
 #include <dt-bindings/zmk/rgb.h>
 
+/ {
+    chosen {
+        // 34 keys.
+        zmk,matrix_transform = &dual_1u_transform;
+
+        // 33 keys. Center two thumb keys replaced by a single 2u key. Remember to adjust your
+        // keymap accordingly!
+        // zmk,matrix_transform = &single_2u_transform;
+    };
+};
+
 / {
     keymap {
         compatible = "zmk,keymap";