Can an employer in PA force an employee to do two positions without asking or compensating in any way?

Unless an employment contract or union rules say otherwise, your employer can generally change things on you. This includes work load, hours, pay, etc. It may seem unfair, but it's not illegal.

Yes, your employer can do this, assuming you are not covered by some type of employment contract or union contract. If not, then you are considered under the law of Pennsylvania an "employee at will" and thus can quit whenever you want or be fired whenever the employer wants. So, in your case, if you don't like what your employer is asking you to do, you can quit.

Unfortuneately, usually this gives all the power to employers. The only thing I can suggest at this point is trying to reason with your supervisors or possibly compromising on what they want you to do. Or even negotiating a time frame that these new duties/hours will last. Good luck!