Can I model a cogeneration unit to supply the electricity and heat demand simultaneously with HOMER?

Yes, you can model cogeneration with HOMER.  To model heat recovery from a generator, set the "heat recovery ratio" to something greater than zero.  That input is on the Fuel tab of the Generator Inputs window.

You can also use excess electricity to serve the thermal load.  To do that, go to the Thermal Load Inputs window and check the checkbox that says "Excess electricity can serve thermal load"

If I want to create a cogeneration plant, can I then connect a generator to the thermal load?

Yes, if you set the generator heat recovery ratio to a non-zero value, then waste heat from the generator will serve the thermal load.  Please look up 'cogeneration' in the index of the help file for more information.