Try this in interactive Python Window, just update the value for lyr.
import arcpy
from itertools import product
from string import ascii_uppercase
lyr =
flds = ["Field_1", "Unique"]
incr = ("".join(i) for i in product(ascii_uppercase, ascii_uppercase))
with arcpy.da.UpdateCursor(lyr, flds) as cur:
for fld1, uniq in cur:
cur.updateRow([fld1, "A{}-{}".format(next(incr), fld1)])