We currently use Outlook 2007 on a laptop. As we change locations frequently and have several PC's and iPads we are considering using hosted email - exchange online plan 1 (or similar) to provide better access to our emails. We have our own domain name and have 2 POP3 email addresses on that domain and the data is currently stored in ONE .pst file on the laptop. If you use the exchange online plans would we count as 2 users and would we each have our own .pst files on the server?

2 POP3 emails addresses in the sense, One Mailbox will have two SMTP addresses?

If One mailbox have two or 3 additional SMTP aliases, it will be countable as one mailbox only

If it is two mailboxes, configured in outlook to deliver the emails to same pst ( not sure it can be done :( ) then it will be countable as two mailboxes

In Exchange you can setup to 20 SMTP on single mailbox,( these are mainly used for outside domains, not company specific), suppose Company "A" has mergence with Company "B", so employees of both company can configure external (non employee at the rate  a.co) OR (non employee  at the rate b.com) to each other SMTP address in Exchange, but DNS (MX record) entry is must in both companies DC.