You can use the index function to select which value to return. So to retrieve the second value from your example you could use:
=index(SPLIT("1.23/1.15", "https://stackoverflow.com/"), 0, 2)
The last argument says which column you wish to retrieve – 1 would retrieve the first value.
Alternatively you could use left / right and find to extract either value from your example. For example to get the first value you could use:
=left("1.23/1.15", find("https://stackoverflow.com/", "1.23/1.15"))