Hi,

When you are saying that there is no pop-up appears, then you mean after clicking the button to show the 1st pop-up window then it doesn't appear, or the 2nd one doesn't appear after you click the button? 

- If the 1st pop-up window doesn't appear, then maybe you didn't return an ir.actions.act_window dict or record to be executed. Then the problem is you either return nothing or True in the method which you call using <button name="method_to_be_called" type="object"/>

- If the 2nd pop-up window doesn't appear, then your first act_window is for a wizard or a model view? If the case is wizard, you need to implement the same method in the wizard. If it's a model view then you should consider checking the action['target'] of the act_window (should be 'new' in this case).

Finally, I suggest you to take a look at the documentation for \ir.actions.act_window

The most obvious reason might be in the decorators you use and what you return in your methods. Both should be @api.multi and should return action dict. Something like:

@api.multi
def main_method_return(self):
    """
    res should be a dict
    """
    self.ensure_one()
    # some calculations
    if self.context("from_side"):
        # some other calculations
    return res

@api.multi
def side_method_return(self):
    """
    res should be a dict
    """
    self.ensure_one()
    # link_to_main_model MUST be real record of main model. Not just empty recordset
    res = self.link_to_main_model.with_context(from_side=True).main_method_return()
    return res