Refer below query.

For SplitString function refer below article.

Split function in SQL Server Example: Function to Split Comma separated (Delimited) string in SQL Server 2005, 2008 and 2012

SQL

declare @value as nvarchar(500), @latitude nvarchar(150), @longtude nvarchar (150)
set @value = '18.489861,73.855066#18.4898497,73.855016#18.4898475,73.8549116#18.490068320047676,73.85491859167814'

SELECT Item 
INTO #Temp
FROM dbo.SplitString(@value, '#')

;WITH CTE AS
(
SELECT SUBSTRING(Item,0,CHARINDEX(',',Item)) 'Latitude',SUBSTRING(Item,CHARINDEX(',',Item)+1,LEN(Item)) 'Longitude' FROM #Temp
)
--SELECT * FROM CTE

Select 
SUBSTRING( 
( 
    SELECT ',' + Latitude
    FROM CTE FOR XML PATH('') 
), 2 , 9999) As Latitude,
SUBSTRING( 
( 
    SELECT ',' + Longitude
    FROM CTE FOR XML PATH('') 
), 2 , 9999) As Longitude

DROP TABLE #Temp

Output