Owner Data Objects

I've a problem.

In my MSSql db I've some tables named username.mytable and only one
store procedure named dbo.sp;


CREATE TABLE [pippo].[mytable] (
[year] [int] NOT NULL ,
[month] [int] NOT NULL
) ON [PRIMARY]
GO

CREATE TABLE [pluto].[mytable] (
[year] [int] NOT NULL ,
[month] [int] NOT NULL
) ON [PRIMARY]
GO

CREATE PROCEDURE [dbo].[sp]
AS
select * from mytable
GO


but when I try to execute dbo.sp (from pippo or pluto connection) I've
this error (users pippo and pluto are owner):

Server: Msg 208, Level 16, State 1, Procedure sp, Line 4
Invalid object name 'mytable'.

How can I access to pippo's (or pluto's) data from dbo.sp?


thanks!!