Don't think there's a built-in shortcut key,
but here's a play to set-up the centring for: CTRL+j
(Try on a spare copy/new book)
Press Alt+F11 (to go to VBE)
Click Insert > Module
Copy and paste the sub CentreAcrossCells
below into the whitespace on the right
'---------
Sub CentreAcrossCells()
If Selection.Cells.Rows.Count > 1 Then
GoTo Pop
End If
If Selection.Cells.Count = 1 Then
Selection.Cells.HorizontalAlignment = xlCenter
ElseIf Selection.Cells.Count > 1 Then
Selection.Cells.HorizontalAlignment = xlCenterAcrossSelection
End If
Pop:
End Sub
'---------
Press Alt+Q (get back to Excel)
In Excel, press Alt+F8
Select "CentreAcrossCells" in the macro list
Click "Options"
Enter a "j" for the shortcut key > OK
Test it out. In any sheet:
Select a single cell with text (say), press CTRL+j
The text will be centred in the cell
Select another cell with text and an empty cell just to the right of it,
then press CTRL+j. The text will be centred across both cells
Note that if you select more than one row, nothing will happen