To do what you want you could do two things:
1) Combine your $conditions arrays and let CakePHP build your new query so you can simply use $this->Model->find() again.
2) Use this. It's an expansion for the mysql datasource that adds the option to do $this->Model->find('sql', array('conditions' => $conditions)) which will return the SQL-query. This option might cause trouble, because for some find calls (especially when you're fetching associated models) CakePHP uses multiple queryies to fetch the associated models (especially in case of hasMany-associations).
If at all possible, option 1 will probably cause the least trouble. Another problem with going with 2 is that if you're trying to combine two queries with conflicting conditions (like 'name = Hansel' in query 1 and 'name = Gretel' in query 2) you will just find nothing unless you plan on writing extra code to parse the resulting queries and look for conflicts..
Going with 1 will probably be a lot simpler and will probably avoid lots of problems.