if object_id('GetResults') is not null
    drop procedure GetResults
go

create procedure GetResults 
    @PassedCriteria varchar(5)  -- i'm just guessing at this length

as

declare @sql nvarchar(max)
declare @params nvarchar(max)
declare @criteria nvarchar(max)

set @PassedCriteria = '%' + @PassedCriteria + '%'

set @sql = 'select Descripcion as Des, CIF as CIF from tabla1 '
set @sql = @sql + ' where Descripcion like @Criteria '
set @sql = @sql + ' union '
set @sql = @sql + ' select Descripcion2 as Des, NULL as CIF from tabla2 '
set @sql = @sql + ' where Descripcion2 like @Criteria '

set @params = '@Criteria varchar(5)'

exec sp_executesql @sql, @params, @Criteria = @PassedCriteria

go

-- now test:
GetResults @PassedCriteria = 'a'