There are quite a few connections between the two symbols and their names. While it's difficult to find instances of @ being called ampersand (for obvious reasons), I have found plenty of examples of it being portmanteau'd into a blend of the two terms.
The earliest reference I can find is from 1996, the ampersat. Another source, Beginning PHP4, in 2000 uses the term every time they refer to the symbol:
For example, ages should be realistic values, addresses should contain
states, zip codes and such like, and e-mail addresses should contain the ampersat (@) symbol.
@ often known as an "ampersat"
According to one source, the symbol's "official" name is the "asperand" (not that I really believe that):
Officially called an "asperand," the at sign separates the recipient's name from the domain name in e-mail; for example, firstname.lastname@example.org.
The Computer Desktop Encyclopedia
Furthermore, Urban Dictionary has entries for "atpersand" and "atpersat".
It's also worth noting that '&' originated as a ligature of et, which is also the name for at in some languages. Ultimately, however, I think it's the visual similarities (both are essentially loopy ligatures), the juxtaposition (shift+2 and shift + 7), and the fact that they both symbolize words starting with "a" that created the connection.