What kind of database are you using?  That can be very important to what kinds of options are available to you.

I don't think Crystal can connect to two different databases simultaneously.  You can do this sort of thing with subreports.  But, that would make calculating your "Full Year" value extremely tricky.

Your best bet is to solve this problem on the data source side.  There are a few options.  Some databases will allow you to do a union or full outer join query, using base tables from two different databases.  You may need to export information into an Excel spreadsheet, or another database, and work with it there. 

Of course, as a professional developer, I can also tell you that maintaining different entire databases for each semester is poor overall design.  It will cause you no end of headaches in the long run.  I would highly recommend adding a column to each of your key tables to indicate which semester it belongs to, and then combine the databases.  But, I do understand that that sort of redesign is not always feasible.

Yes.  You can create a query in Access that will reference another database.

Perhaps a better option, though, is to create a third Access database.  In that database, create linked tables to each of your previous databases.  (Creating linked tables is a considerably easier and more foolproof function that trying to reference another database in a query.)  Among other things, this will allow you to intelligently alias your tables (e.g., Spring_Enroll, Fall_Enroll), so you can keep up with what you are doing.

Then you can create a number of queries, combining the data any way you want.

(I do want to throw out another plea for redesign, though.  Note that, if you want to combine three semesters of data, you have to add in the linked tables of the third database, and redesign all of your queries.  Doing the above will get you a quick solution for now.  But, I recommend that you also take some time to redesign the database from scratch.)

Use an Access UNION ALL query to combine the two data sets into a single data set.  Then, use that query as the data source in your Crystal report. 

By the way, Crystal can connect to 2 data sources.  The only restriction is that a join is limited to a single field in such a case.

- Ido