Add values from two existing column into new columns for a total
Just change your table create statement to use a computed column.

CREATE TABLE [dbo].[Incomes](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [ClientIncome] [float] NOT NULL,
    [PartnerIncome] [float] NULL,
    [HouseholdIncome] AS COALESCE(ClientIncome, 0.) + COALESCE(PartnerIncome, 0.)
)


If your table is already created, you can add a computed column:

ALTER TABLE dbo.Incomes ADD HouseholdIncome 
    AS COALESCE(ClientIncome, 0.) + COALESCE(PartnerIncome, 0.);