The image appended clarify the question.
There is no problem if Vccy is 1V greater than Vcca.
The one thing you might want to consider is the power-up sequence of the device. In the image attached, at time = 0 to time = 0.25 us, Vcca is greater than Vccy hence will cause damage to the device. Please be reminded that the recommended power-up sequence is Vccy first then Vcca. The power-up sequence is best explained at the Power Supplies Section of the ADG3308 datasheet, page 16.
Let me know if this helps.
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