Is it possible to add multiple print templates to our invoices from a custom form? Or from another

I'm trying to add an option on our custom Invoice forms to be able to print one of two types of pdfs: a basic pdf template and an advanced pdf.  I obviously can't select multiple templates from the custom Invoice form:

On our sales orders, a while ago our consultant added "Print Picking Ticket" as a drop down option when printing sales orders:

This "Print Picking Ticket" actually is part of a deployed UserEvent (AfterSubmit) customization that generates a pdf from html stored in a custom field on the sales order, so it's very different from what I'm trying to achieve on the invoice records.  But interestingly I didn't see anywhere in the deployed UserEvent script (no /N/ui module was loaded and no addButton function) where "Print Picking Ticket" is added, so therefore it must have been added through another way in Netsuite?  I didn't see it on the custom Sales Order form either.