I wouldn't double the dose that's more than you need, you could drink half of the bottle then add in the miralax, it's okay if you have a little more than 8 oz of water.

Water bottle is 19.9 FL OZ and miralax says to mix the dosage with 8 OZ so do I double it or is it ok to put it in more water? @maya.rose

You can put it in a water bottle and shake it or pour it in a glass, add water and stir.