First of all, overall
database size has no impact on overall program performance and startup time. AllMyNotes is loading
only single note content in to the base memory - currently edited/viewed note. It will not parse all notes even on performing global search. It's equally fast to start to and to work no matter how many notes you have. At this point I see no sense in breaking your big database to many. And 500Mb is not that big size - many user have databases bigger than 2 Gb

Regarding performance I can give you only two recommendations:
1. Try to
keep individual note size under 50-100Mb limit, this way it will be fast enough to open individual notes. Major spaces eaters are big images and attached files. Images are compressed, so basically need to story big data files not as attachments but as hyperlinks to local files, AllMyNotes supports this kind of hyperlinks.
2. The
backup time may be relatively big for big files. If this an issue you can decrease automatic backup interval in backup settings.
Now, if from some other reason you still need two or more databases. It's easy to create two or more databases using menu item File -> New. To
open them all simultaneously just double-click on them in file file explorer. To
move data between databases you can use
clipboard or drag&drop. I recommend to copy not everything at once but by copying small pieces (folders) as the clipboard size is limited and may not fit all the unpacked images (for example, for 100Mb of notes containing images it may require 2Gb of ram).
To open certain DB(s) on
Windows startup just put a shortut to your .ddb data file(s) in to the Start->Programs->Startup folder.