Unfortunately, ACT does not allow for multiple databases to be opened at once. Is there a reason you need more than one database? It is far easier to work in 1 database, and segment the database using certain fields, than to manage multiple databases and calendars.
One thing that I would suggest looking at is using the dynamic groups feature. This allows you to create a relationship between information entered in to a field and the groups that you have created.
Example: I created a field called Contact Activity Status. I built a drop down list inside of the field that one could select either "active" or "inactive".
After I created the field I then went into my groups section and set up a group called Contact Activity Status and then set up two sub-groups below which were called "active" and "inactive".
Once you have set up the groups you can click on the button that is named "Add/Remove Contacts" which is located under the notes and history tabs for your group.
Once you have clicked on this it will bring you to a new screen where you can either click on a "contacts" button or a button named "edit criteria".
In order to set up a dynamic relationship between information typed in to the field and the group that you want the contact to be associated with you need too click on the "edit criteria" button.
Once here ,set up the rules for your query. Example: In order to get my contacts, that I selected as active, to be automatically entered into my Active Group I set up a query that reads like this.
Type Field NameOperator Value
Contact Contact Activity Status equal to Active
With this query set up any time I enter in a Contact and I select this field as Active the contact is automatically added to this group. I hope this information is useful. This is just one example out of thousands of different ways that you can manage all of your contacts under one database.