If you can identify a separator character (e.g. '~') that is never produced as output by the encrypt method, then you can use that:
const char sep = '~';
string combined = encrypted_string_1 + sep + encryptedstring_2;
And then split it later:
string[] parts = combined.Split(sep);
which can then be decrypted.
Edit:
The output of Encrypt() is already base-64 encoded, which produces[1]...
uppercase characters "A" to "Z", lowercase characters "a" to "z", numerals "0" to "9", and symbols "+" and "/". The valueless character, "=", is used for trailing padding.
So any other character can be used as a separator.
[1] see https://msdn.microsoft.com/en-us/library/dhx0d524(v=vs.100).aspx