It sounds like you're trying to take the higher number and increase it upward to the nearest integer and then subtract that same amount from the smaller number.
Example:
84.75 and 0.32 would give you 84.75 + .25 and 0.32 - 0.25, which in turn gets you 85 and 0.07.
If that's the case, you could do the following:
double highNum = 84.75;
double lowNum = 0.32;
double newHighNum = Math.Ceiling(highNum);
double newLowNum = lowNum - (newHighNum - highNum);
Console.WriteLine($"Started with: {highNum} and {lowNum}" );
Console.WriteLine($"Ended with: {newHighNum} and {newLowNum}");