The second part of the formula by itself highlights everything red only if DateTwo has something in it (not Null) ...That sounds wrong. As far as I can tell, you should only be getting red when DateTwo is null. I don't see how you could be getting red when DateTwo is not null.
Create your formula
Name - HighlightOld
If isNull({DateTwo}) and ({DateOne} < DateAdd("d",-5, CurrentDate)) Then
'Y'
Else
'N'
Right click the field
Click HIGLIGHTING EXPERT
Click NEW
In the VALUE OF dropdown select the formula
Set the comparison to IS EQUAL TO
Set the value for comparison (bottom dropdown) to Y
Select the BACKGROUND COLOR
Click OK
You can do it in the FORMAT FIELD - BORDER TAB
Add a formula to the BACKGROUND
If isNull({DateTwo}) and ({DateOne} < DateAdd("d",-5, CurrentDate)) Then
crRed
mlmcc