Vitamin C is stored in the cells so if you take it separate to Miracle-Mineral-Supplement and if it has time to be absorbed from the blood into the cells and the tissues then it won't interfere with Miracle-Mineral-Supplement too much and will protect the cells from oxidative stress.
Also it may be present in the blood mostly in it's oxidized state(not sure about this)
Since vitamin C is reduced back from its oxidized form when it gets into the cells it is not destroyed by Miracle-Mineral-Supplement (ClO2) so I don't see how there is need to take more than usual.
I take about a gram or two of sodium ascorbate once and about 50 % of this is supposed to be absorbed.
Taking several grams a day in several doses may be a good idea to build up to a
short term use of higher multiple doses of MMS though as in the case of heart problems for example. unless you already had been doing so.