Not all fuels mix well. However, in the case of white gas and unleaded gasoline, one is basically a (much) cleaner version of the other, so you're not mixing so much as diluting the white gas with its inferior (for cooking) sibling.
Still, you would get better performance out of the remaining white gas if you don't mix it with the unleaded. There's really nothing stopping you from carrying the gasoline in a lightweight bottle (say, a plastic water bottle, or an old nalgene. Just be sure to mark it very well and dispose of it responsibly afterward to make sure nobody drinks from it) and filling it into the stove fuel bottle after you've used up the clean white gas.
Another issue with using fuels other than white gas in the whisperlite is that they burn dirtier. There is more carbon buildup in the stove, and you will likely have to service it or unclog it sooner with more usage of less clean fuels. So make sure you have the tools and the know-how for disassembling, cleaning, and reassembling your stove in the field (obviously, you should only disassemble the parts that are clogged in the field, and then do the complete service at home when you return from your trip.)
Now, the caveat emptor with burning gasoline in your camp stove is that this gasoline isn't really designed for cooking and heating, but is intended primarily for automotive purposes. As such, it has many additives and dyes in it that are toxic and or dirty, and you should make sure to minimize skin contact and inhalation of fumes as much as possible. More importantly, wash your hands with soap after handling the stove and gasoline, and ESPECIALLY BEFORE HANDLING FOOD!
Then again, the above precautions should be taken with regular white gas as well... So yeah.