Pennsylvania is one of about 20 states nationwide that allow stacking. Stacking uninsured / underinsured motorist policies is an option that allows you to increase the limits you select for your UI/UIM bodily injury coverage. Limits increase based on the number of cars you are insuring. Keep in mind this increased level of protection typically will raise your insurance premium.

An example of stacking is Jim has limits of $100,000/$300,000 for his uninsured motorist bodily injury coverage and is insuring 2 vehicles. If he leaves them "unstacked" Jim's limits would stay at $100,000/$300,000. If Jim instead chooses to "stack" his UMBI coverages then his limits would double to $200,000/$600,000.

Since "stacking" is the application of more than one policy limit to the same loss or occurrence, if you only have one vehicle on your policy than there would not be coverage to stack. With a single car policy there is not multi-coverage to stack thus no benefit since this option would not be available to you.

Typically, you cannot purchase the stacked option for UM/UNDUM coverage on a one vehicle policy, because there is no stacking on a one vehicle policy.

If there are multiple policies, it may be possible to purchase stacked coverage if there is only one vehicle, but multiple vehicles in the household. For example, 2 cars on 2 different policies or multiple policies in the house, say owned by a spouse. Otherwise, the statute lists stacking available with multiple vehicles.

Title 75 §1738 Pennsylvania Consolidated Statutes Chapter 17 reads:
§ 1738. Stacking of uninsured and underinsured benefits and option to waive.
(a) Limit for each vehicle.--When more than one vehicle is insured under one or more policies providing uninsured or underinsured motorist coverage, the stated limit for uninsured or underinsured coverage shall apply separately to each vehicle so insured. The limits of coverages available under this subchapter for an insured shall be the sum of the limits for each motor vehicle as to which the injured person is an insured.
(b) Waiver.--Notwithstanding the provisions of subsection (a), a named insured may waive coverage providing stacking of uninsured or underinsured coverages in which case the limits of coverage available under the policy for an insured shall be the stated limits for the motor vehicle as to which the injured person is an insured.
(c) More than one vehicle.--Each named insured purchasing uninsured or underinsured motorist coverage for more than one vehicle under a policy shall be provided the opportunity to waive the stacked lises such waiver shall be reduced to reflect the different cost of such coverage.
(d) Forms.--
The named insured shall be informed that he may exercise the waiver of the stacked limits of uninsured motorist coverage by signing the following written rejection form:
2. The named insured shall be informed that he may exercise the waiver of the stacked limits of underinsured motorist coverage by signing the following written rejection form:
(e) Signature and date.--The forms described in subsection (d) must be signed by the first named insured and dated to be valid. Any rejection form that does not comply with this section is void.

Some specific carrier rules when more than one policy exists in the household:
Stacking -- Two or more policies issued to you, your spouse or a resident relative can be added together when damages exceed the limits on one policy. If you have only one car in your household or if your car is insured in the name of a corporation or business, the additional benefits provided by the stacking option may be very limited.
Non-Stacking -- If an injury occurs in a vehicle owned by you, your spouse or resident relative, only the coverage on that vehicle would apply. If you have only one car in your household or if your car is insured in the name of a corporation or business, the additional benefits provided by the stacking option may be very limited.

With this, you learn that if there is another policy in the home, he/she may be able to stack the benefit. It is not the general rule to be able to stack without more than one vehicle. 

Please add a comment if you know of a carrier with that option.