Why is the message I send split into two messages?

This happens due to concatenation.
A standard message can contain 160 characters. Messages containing more characters will be split up. The length of the split messages will contain max 153 characters.
A Unicode message can contain 70 characters. The length of the split Unicode message will contain max 67 characters.
NOTE: Not all destinations accept/allow concatenation!

See also: https://en.wikipedia.org/wiki/Concatenated_SMS