The Rebbe responds to a question about Yom Tov Sheini observance by visitors from Israel, noting that while it's a matter for a halachic authority, he has seen individuals from Israel observe only one day when their intent is to return, provided it’s not done publicly. Though not heard directly from the Rebbe Rayatz, this was done with his knowledge.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/nissan/973