problem with 'IF' function

Hi guys,

Using Excel 2007.

I’m trying to build a tool which will show shut down start, finish, and duration in a holistic view so conflicts between sites can be identified.
It will also audit the relationship between SAP revision dates and the actual scheduled dates.
It has inputs from a Primavera excel extract on one sheet.
I can get the cell I want populated to show me the start and finish dates of the shutdown, but I want the dates in-between populated so I can conditional format the text to show a coloured view.
So
To get column J populated, I used the following formula
J518=G518&H518
G518 =IF(ISNA(VLOOKUP(E518,'Primavera Start'!$A:$B,2,FALSE))," ",VLOOKUP(schedule!E518,'Primavera Start'!$A:$B,2,FALSE))
H518= =IF(ISNA(VLOOKUP(F522,'Primevera Finish'!$A:$B,2,FALSE))," ",VLOOKUP(schedule!F522,'Primevera Finish'!$A:$B,2,FALSE))

Ideally, what I want J518 to be is =if(G518&H518=””,G517, G518&H518
So
If G518 and H518 is blank, show me the cell above, if not blank show me G518&H518.
But for some reason, regardless if cell G518&H518 is blank, it shows it as populated.

Any help will be great.

If I can pull this off, I will dance!

Please post a sample workbook:
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Try:

=IF(AND(ISBLANK(G9),ISBLANK(H9)),J8,H9&G9)

Edit: sorry that won't work, try:

=IF(AND(LEN(G8)=1, LEN(H8)=1),J8,H9&G9)