In some languages, e.g. in Italian, prepositions may appear fused with articles in a unique graphical form. EAGLES encourages the treatment of these forms as separable, therefore having two tags corresponding to the two different categories, but also allows the use of a unique tag.
This general recommendation, i.e. to separate different categories and to encode them with different tags, is also applicable to other cases of contractions, such as negation, cliticisation, etc.
In general, there will be language-specific options for the treatment of these phenomena.