January 11, 1756
Family History Library in Salt Lake City film #6182
Both the first and second editions of "Ancestry and Descendants of John Walker show Simeon Walker's date of baptism as 11 Jan 1755-56. It looks as though there is some confusion about the double dating that was used for dates that occurred between January 1 and March 24, prior to the calendar change in 1752. Prior to the calendar change, dates that occurred between January 1st and March 24 were written as double dates, such as 1731/32. The system of using double dating stopped in 1752 - Simeon was baptized long after that.
It is clear that Simeon Walker's baptism occurred on 11 January 1756. Perhaps the confusion comes from the page in the church records where his baptism is recorded. At the top of the page is clearly written 1755 6. The recorder is not using double dating here, just showing that baptisms on that particular page occurred both in 1755 and 1756.