Firstly - forgive me for this but I'm just trying to prevent any confusion - but I think what you're referring to in your post as a 'db' is in fact an instance of SQL server. When the ePO installer runs it is seeing that there is already an SQL server on the machine, so it does not prompt you to install another one. When it gets to the point in the install where it presents you with the list, this is actually a list of SQL servers that is can see - it's asking you which one of these you want it to use: it will then create the ePO database on this server.
So the question is - do you want to create the ePO database on the SQL instance being used by Act? I'm not familiar with Act so I can't say if the way the Act SQL instance is configured will cause ePO any problems (as things like case-sensitivity can have an effect.) If you do, then simply select the Act instance from the list, and ePO will create the db on this instance.
If you want the ePO db to be on its own separate instance, you'll need to manually install an instance of SQL first, and then choose this instance during the install of ePO. The instructions in KB 52109 should help here.
Horribly late reply (lost track of the entry and lots of time...), but many thanks - that was the simple solution.