Save illustrator file as tiff along with spot colours as spot chanels

My DTF machine accepts only tiff images. I have staffs name in illustrator. DTF demands to create on channels of the file-one for varnish and another for printing white. So when I keep two spot colours of the file and select over print then export to tiff, i am not getting spot colours as extra chanels. I am getting only CMYK. how to save, so that I get spot colours as extra chanels in tiff format