How to split custom device symbol into separate parts in Eagle (within a single footprint)
Simply create all of the smaller symbols that you want, and then in the device editor, simply add multiple symbols in the same way you would add a single one.

Eagle is quite happy for you to have multiple symbols for the same part. You can name each one in the device editor and that name will be suffixed to the name in the schematic.

For a simple example of this, have a look in the 40xx library (an Eagle default library). You can see how there are multiple gate symbols in the same device all connected to a single package.

When a part has multiple symbols, you can move the symbols around independently of each other, so can place them wherever you want in the schematic. You can also (assuming your license allows) use multiple sheets within your schematic and place different symbols in different sheets.



In your additional question, to split a trace across sheets, you draw a short wire connecting to the pin on the first sheet, then give that wire a name (the name of the net). You can also use the label tool to add a text label to the wire which automatically reflects any change to the net name.

On the second sheet, simply do the same thing. Any wires in a schematic with the same net name are considered connected.