Hello - is it legal in Pennsylvania to have two different rates for regular pay and overtime pay?

For example: Regular pay is 12/hour

Overtime pay is based on 10.50/hr at time and a half
So overtime rate is 15.75, instead of what WOULD be 18/hr
at the regular pay rate.

Confused.

Not legal as stated. Let's try an example. Bob works 50 hours this week. 30 hours at $10/hr and 20 hours at $12/hr. That means before the OT premium Bob is paid $300+$240=$540. That means that the Regular Rate of Pay is $540 / 50 = $10.80/hr. RRP is defined in law and not something that the employer gets make up. To complete this example, Bob is due (in addition to the $540 in base wages), an additional premium pay of 10 hours x 50% x $10.80/hr = $54.00.

http://www.dol.gov/esa/whd/regs/compliance/whdfs23.pdf

Where an employee in a single workweek works at two or more different types of work for which different straight-time rates have been established, the regular rate for that week is the weighted average of such rates. That is, the earnings from all such rates are added together and this total is then divided by the total number of hours worked at all jobs. In addition, section 7(g)(2) of the FLSA allows, under specified conditions, the computation of overtime pay based on one and one-half times the hourly rate in effect when the overtime work is performed. The requirements for computing overtime pay pursuant to section 7(g)(2) are prescribed in 29 CFR 778.415 through 778.421.

OP, do you perform two different types of jobs a different hourly rates of pay?

No, the job is consistently the same work and same rate of pay. The raise to 12 dollars was given due to a promotion. however, she was told that any overtime would not reflect the raise, and would be based on the old rate at 10.50/hr. So 40 hours will be calc'd at 12/hr, and anything over will be paid 15.75/hr (or time and half at 10.50)

Not legal. Overtime is a calcuation based on actual hours worked and actual money earned in the pay period. The employer cannot legally try to uncouple the OT calculation from the actual hours/dollars earned in the workweek.