Say a witness is presented and is testifying in a Bucks County criminal prosecution. Then, the witness begins testifying to what they overheard in a phone call with another person. This involves double or multiple hearsay and is part of Pa.R.E. 805.
Essentially, hearsay within hearsay is only admissible if each statement is within an exception of the hearsay rule. This is a well-established rule regarding independent exceptions (see Commonwealth v. Scott from 1983). So, even if one of the statements has an exception that could be admitted, the other must also have an exception.
Now, if there are more than two levels, there is a bit of diminishing marginal returns. Bucks County criminal defense lawyers would be wise to review Commonwealth v. Peay of 2002 that states, "experience suggests an inverse relationshiop between the reliability of a statement an the number of hearsay lawyers it contains," where the case cites U.S. v. Fernandez, 892 F.2d 976 (11th Cir. 1989).