Sought advice from colleague.
This worked (double quotes encasing a space)
[Copy of Obj Acct] == " " ? [Copy of Sub] : ([Copy of Sub] == " " ? [Copy of Obj Acct] : [Copy of Obj Acct] + "." + [Copy of Sub])
This didn't (double quotes encasing no space)
[Copy of Obj Acct] == "" ? [Copy of Sub] : ([Copy of Sub] == "" ? [Copy of Obj Acct] : [Copy of Obj Acct] + "." + [Copy of Sub])
Now the concatenation is working as intended, with no period if either [Sub] or [Obj Acct] column is empty.
--I will need to evaluate why the excel column [Sub], when containing nothing is importing to SSIS as a space....