I'm pretty sure Chile meets this criterion OTL given that it owns all of the north coast of Tierra del Fuego and a small strip of the east coast. But if that isn't enough, then all you need is for the negotiations that led to the border treaty of 1881 to produce a slightly different result and you could have Chile either with a clear Atlantic coast or Argentina with a Pacific one.
Since the whole point of the 1881 treaty was, from the Chilean side, to get direct Atlantic access however small, I'd agree with you.