To elaborate on Dave's succinct answer... no, hsmtp cannot be configured to use more than one email server at a time, but I suspect that the problem you have is that half the users you want to notify are on Outlook and half on Lotus Notes.
Assuming that the users are able to send emails to one another between the two systems, then they must have some kind of external email address that they use for this which will route the email to the correct system - if so, then configure Harvest to talk to the external SMTP gateway and set the email addresses to the external email addresses. You then don't need to know which system the user is on.
If you can't do this, then the only way I can think of doing this is to front hsmtp with a script or program that identifies the email addresses as being Outlook or Lotus and forwards them to a hsmtp after first setting HARVESTHOME and CA_SCM_HOME to a location containing a hsmtp.arg setup for each mail server (you will need two such folders). This is a bit fiddly as you will also have to deal with passing standard input to hsmtp.
Best regards,
Robert