What order do split TXT records get assembled in?
The parts are always reassembled in order. The 255 character limit is on parts of the TXT record. Without EDNS0, there is an addtional 512 byte limit for a UDP response. However, this does not apply to a TCP response.
Also, you may encounter a problem with AWS Route53, where
IN TXT "v=spf1 .... first""second string..."
IS treated as an equivalent to
IN TXT "v=spf1 .... firstsecond string..."
But if you use spaces... the following WILL FAIL:
IN TXT "v=spf1 .... first" "second string..."