If you have only one target for a source, please try doing the following:
1. Create a dictionary (e. g. English => Spanish).
2. Export it into an XLS file (select Export => XLS in the AnyLexic main window).
3. Import the previously exported file into AnyLexic once again (select Import => XLS in the AnyLexic main window). When importing, swap the source and target columns, so that Source will become Target and vise versa. You will get a Spanish => English dictionary.
Detailed information about export/import operations you may find in the AnyLexic help file.
An alternative solution:
You may create only one dictionary (e. g. English => Spanish) but in order to find a Spanish word use Advanced Search.
Click the "+" button and select "target".