So long as only one of the two columns has a non missing value, you could try 'Sum()'. Look at the formula in the table below:
New Table( "Both",
Add Rows( 4 ),
New Column( "a",
Numeric,
"Continuous",
Format( "Best", 12 ),
Set Values( [., 1, ., 3] )
),
New Column( "b",
Numeric,
"Continuous",
Format( "Best", 12 ),
Set Values( [., ., 2, 4] )
),
New Column( "c",
Numeric,
"Continuous",
Format( "Best", 12 ),
Formula( Sum( :a, :b ) )
)
)
e