Just a quick reactive thought, untested ........

Use same type of programming in both elements
=====================================

On Enter Element program
Save current value as global variable

On Exit element program
If other element has a value send message confirming use this vs that
If confirmed, clear other value and finish

If not confirmed restore initial value using global variable
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Might also want to consider a message On Element Entry
If other element has value, message asking to confirm entry here to replace the other one.
React based on confirmation.

Might also want to consider changing element Visibility and ReadOnly states based on conditions.  Need more information about how you are using these elements.  If both are empty then both are wide open.  If one is not empty then the other one is locked out.

Captain,

You could also try this:

// Place this in the "A :: On Element Immediate Change" event
If NOT @IsBlank(B)
	{
	@MsgBox("You cannot have a value in both the A and B elements",
		"",
		"If you want a value in this element, first delete the other one.")
	Clear(ThisElement)
	}

// Place this in the "B :: On Element Immediate Change" event
If NOT @IsBlank(A)
	{
	@MsgBox("You cannot have a value in both the A and B elements",
		"",
		"If you want a value in this element, first delete the other one.")
	Clear(ThisElement)
	}
 

Carl, that's working well, thanks again.  Now I need to check against the total, and for this I think I need to go to Bob's idea of setting a Global Variable.  I'm having some trouble figuring out how to do this.  I'm hitting the books as I type this, but any hand-holding anyone can provide would be appreciated.

Here's what I'm trying to do.... A picture of my form can be seen at http://www.jamiara.com/invform.jpg  As you can see, the Vehicles and Labor sections have two columns to the right of the description lines.  The column on the left, in yellow, total up into the Totals line (under the gray elements).  If any yellow element has an entry, the gray element to the right is disallowed, and vice versa.  That's working, thanks again Carl.

The Total element (under the gray) should be manually editable UNLESS any of the yellow elements above have an amount, in which case editing is disallowed because it works as an automatic total of all the yellows. 

So I need to set the Global Variable equal to the value of the yellow total element.  If any of the left-column yellows have an entry, its value will be <>0.  If that's the case and a user tries to edit it I need to pop up a message box saying "no way" and reset the value back to the global variable.  If none of the left-column yellows have a value, then the total box can be edited any way the user pleases.

Conceptually, I think I've got it straight, and even as far as which on-element action I think I'm clear.  But the specifics of setting a global variable and then checking against it are not in my toolbox yet.

Thanks in advance for any help.