I have a cdt which have two fields [field1=,field2=150],[field1=230,

Why not create an expression that checks which value is available and then shows that. You can apply this to your list of CDTs.

fn!apply(rule!customRule(_), ri!array)

customRule:
fn!if(
fn!isnull(
ri!array.field1
),
ri!array.field2,
ri!array.field1
)

This gives you an array and map the same to the grid field.

i dont want to update the cdt.. only while displaying in grid i need to apply

Here you are not updating the original CDT, store the result in the temporary variable and map to the grid

may be you can try like this, write a expression like if(isnull(field1),field2,field1)
Then you do this function in apply like apply(fn!rulesaidabove,merge(field1,field2))
May be you need to do a little work around if the merge is not working, Hope this would help!!! :)

If you have Appian common objects app deployed, there's a rule you can use
APN_replaceNull (Returns nullableValue only if it is not null. Else return replacementValue.)

Use apply on the grid it will only display and not update CDT.