John's correct, here's another way:
SELECT
col1 = MIN(CASE WHEN t.value = x.min_value THEN value ELSE NULL END),
col2 = MIN(CASE WHEN t.value = x.min_value THEN t_stamp ELSE NULL END),
col3 = MIN(CASE WHEN t.value = x.max_value THEN value ELSE NULL END),
col4 = MIN(CASE WHEN t.value = x.max_value THEN t_stamp ELSE NULL END)
FROM @t t
CROSS APPLY (
SELECT min_value = MIN(value), max_value = MAX(value)
FROM @t
) x
WHERE t.value IN (x.min_value, x.max_value)
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden