[exim] Using multiple SMTP AUTH drivers

Hi all,

I have successfully set up Exim with Dovecot for a mail server, and I'm
using SMTP AUTH with driver=dovecot so that I can authenticate with
Exim by using the same password as for the corresponding Dovecot
account.

However I have some network devices (UPS, NAS, etc.) that I would like
to be able to send mail through the server as well, and as these
devices are behind dynamic IPs I want to use SMTP AUTH before allowing
them to submit mail, since I can't whitelist their changing IPs. They
don't have (or need) IMAP accounts, so I'd like to set up a separate
set of credentials just for SMTP from these devices.

Unfortunately if I add a new auth section to the Exim config with
driver=plaintext, I get an error:

two server authenticators (dovecot_login and file_login) have the same
public name (LOGIN)

Is there some way to configure multiple authentication drivers with
Exim, such that they are tried in order until one is found that
succeeds?

Many thanks,
Adam.