Lets take Combo1 & Combo2

If u r selecting < Item1 > from Combo1 the same item1 
should not be present in the Combo2 right..
if it is the case

step1) select Item1 from Combo1
Step2) write a function to validate the given item i.e 
passed as perameter(Item1)by getting the innertext of the 
Combo2 and use Instr function to find Item1 is there in the 
inner text..if no Pass...If yes Fail.........


if the case is
the item selected in the combo2 should not be <item1>
then

get the selected item from the Combo2

strcomp("selected item","item1")

if yes fail
if no pass.

suppose in our flight reservation window "Fly from" 
and "Fly To" comboboxes values should not be same.ok.
In taht situation below code will give all fail status 
because Our flight reservation window has both values same 

Just understand the code and implement it on ur application

SystemUtil.Run "C:\Program Files\Mercury 
Interactive\QuickTest 
Professional\samples\flight\app\flight4a.exe","","C:\Program
 Files\Mercury Interactive\QuickTest 
Professional\samples\flight\app\","open"
Dialog("Login").WinEdit("Agent Name:").Set "rayudu"
Dialog("Login").WinEdit
("Password:").SetSecure "49192ea21d0ec541ad12119e494c01ba2bb
d510b"
Dialog("Login").WinButton("OK").Click

a=window("Flight Reservation").WinComboBox("Fly 
From:").GetROProperty("items count")

b=window("Flight Reservation").WinComboBox("Fly 
From:").GetROProperty("all items")

a1=window("Flight Reservation").WinComboBox("Fly 
To:").GetROProperty("items count")

b1=window("Flight Reservation").WinComboBox("Fly 
To:").GetROProperty("all items")

spl=split(b,vblf)
spl1=split(b1,vblf)
For i=0 to a-1
val=spl(i)
msgbox val
val1=spl1(i)
msgbox val1
If val=val1 Then
	reporter.ReportEvent micFail,"Step1","Test case is 
fail"
	Else
	reporter.ReportEvent micPass,"Step1","Test case is 
pass"
	End If
Next