bcp will always take the first result set it finds in the stored procedure, you cannot overcome this.
What you could do, is create 2 procedures, or append the outputs using a union all, to create 1 file with the 2 datasets in it.
create proc proctest as
begin
select 1,2,null
union all
select 1,2,3
end
This is the only way you can overcome this.
You can also create 2 files, with 2 bcp commands, and merge the files together using a dos command with the xp_cmdShell.