This is really simple and most people probably won't realize this, but the jist of it is if you have a Unique item that you can only have one of, or you just want your MH/OH to look like your other weapon you can just;
1. I want my MH to look like my OH
2. I unequip my OH
3. I transmog my MH into my OH
4. I equip my OH back
And there you go. Trying to simple drag your OH into your MH or vice versa in the transmog window does NOT work.