Can a patent owner share his patent with another party? like 20% to be
owned by the original owner and and 80% sold to another person?  Is
there a legal term for such arrangement?

If it's doable, can one party exercise the patent rights independently
without consent from the other party?

The process you describe is called assigning patent rights.  It is
done by contract which is completely separate from the original
patent.  It is extremely common that employees who develop products as
part of their job are require to assign full patent rights to their
employer who paid for the work leading to the patent. What you want to
do is similar in concept. It sounds like you wish to sell partial
patent rights to an investor.  It's very easy.  Just have a lawyer
prepare a contract stating the obligations of both parties.  If there
is a dispute later, the contract will then become the basis for a
lawsuit asserting that the offending party renegged on a contractual
obligation.

Thanks, Bigblue .. I think it's not difficult to understand two people
can share the agreegated value of a patent. But in term of practicing
the patent, it's not like I can use 20% of the right to produce 20
pieces of parts, and have the rest of 80 pieces pruduced by another
party who has 80% right, to make a product that is composed of 100
parts.

Can the 20% owner go ahead and produce the product without the consent
of the 80% owner, and later on compensate the 80% owner by appropriate
80% of the income to him/her?  Does the 80% owner have the say in the
selling price,  bacause he/she does have a vast interest in the IP?

Patentdog,
That is the sort of thing the contract will/should spell out.
Normally, one would not expect the partners to both be indendently
producing and independently selling superwidgets.  That is inefficient
production and marketing.
The usual situation would be the inventor/patent holder allowing a
producer to make and market the items with the contract defining the
patent holder's share of profit, total sales (maybe), or a fixed fee
that would insure him an income regardless of whether the things were
produced or sold,
OR a combination:  a fixed fee plus something related to sales/income.

HOpe that helps a bit.

Hi

The previous information is slightly incorrect.  The USPTO will only
record assignments for 100% ownership of a patent.  If an assignment
is given to 2 or more people/companies, EACH owns 100% of the patent
and can sell, license or practice the entire patent WITHOUT consent of
the other Assignees.

What you want to do as the owner is LICENSE the patent to a
person/company and spell out exactly what the license gives that
person/company.  The license can be exclusive or nor-exclusive, allow
the practice of only a portion of the patent or the whole patent or
any number of other arrangements.  Licenses are not recorded with the
USPTO and are ruled by contract law of that state.

Thanks to all who has provided his/her valuable comments, I just got
this one out from the USPTO website.... can anyone explain? especially
in the regard of "In the absence of any agreement to the contrary"

35 U.S.C. 262 Joint owners. 
In the absence of any agreement to the contrary, each of the joint
owners of a patent may make, use, offer to sell, or sell the patented
invention within the United States, or import the patented invention
into the United States, without the consent of and without accounting
to the other owners.

This section of the Code is stating what I wrote above.  Each person
(inventor) or company who is an owner (assignee) owns 100% of the
patent and can act without the consent of the other owner, IF there is
no agreement saying otherwise.  For Example, if 3 inventors are listed
on the patent and they happen to work for two different companies or
universities, and each inventor assigns his/her patent rights to their
company or university, then EACH company/university owns 100% of the
patent.

Let's say Company XYZ wants to get a license to pratice the patented
invention, then XYZ can go to either owner of the patent and purchase
a license.  The owner who sells the license gets to keep ALL the money
from that license.  If both owners are selling a license, then XYZ can
go to the lowest seller.