Per the roadsync faq it appears to be a little mis-leading. What they actually mean is that you'd have to re-enter the settings (e.g. your username and password) into the RoadSync settings to 'switch between accounts'. Essentially you're re-configuring RoadSync.
Does RoadSync support multiple Exchange mailboxes? RoadSync does not support multiple Exchange mailboxes email accounts at the same time. However, you can switch between accounts by changing your settings in the RoadSync application.
(via: www.dataviz.com/products/roadsync/series60/faqs.html#mailboxes)
You could install the latest nokia mail for exchange for your second account. I haven't tried specifically, but I do know I've had them both installed on the phone at the same time one set to Manual sync, one set to Automatic. Without syncing contacts, calendar, tasks, etc. The latest MfE supports sub-folders and HTML e-mails which is a welcome addition. I don't know what this would do for battery life, etc, however.