Basically, a sandwich is some food contained between two slices of bread. Thus, both of your versions sound odd. They sound like you are asking for "a slice of two slices'. It sounds like you want me to take a sandwich and cut off (ie slice off) a small piece for you.
I think you probably just mean 'May I have a sandwich, please?' This is the normal expression. If you want more than one sandwich, say 'May I have some/two/three sandwiches, please?'
A part or a piece (instead of a slice) would probably work if you want somebody to share their sandwich with you.
https://www.google.com.my/amp/s/www.englishforums.com/English/ASlicePieceOfSandwich/znhmc/amp.htm