If someone drinks one drink for 2 days in a row, will blood peth test be positive 8 days after last drink

No. Phosphatidylethanol (PEth) is detectable only following consumption of about 6-7 standard drinks within a one-week period. Some estimate higher at about 200 g of alcohol.

At any rate PEth is not detectable following a couple of drinks and is used to assess sustained drinking rather than infrequent low level intake.

If you don't mind, I'd love to know the type of agency offering/requiring this test.

Ref.
USDTL
Hartman, et al 2007