I figured this out earlier while farming Uldum and needed to repair. I had brought my alt there earlier that had vendor mammoth.
Here are the steps:
1. Move your main and alt to same spot.
2. Log-in your main with vendor mammoth and mount up
3. Open second WoW instance (recommend to use windowed-mode and reduced resolution)
4. Log-in to your account again and this time sign on your alt
5. Your main should still be mounted with vendors available.
I have used this same method to open portals for my alts. Only trick here is that you have to first group them together by having a second person invite both your main and alt. Enjoy ;o)