Greetings,
This is possible however the long string of text will be stored in the database
twice, once on each list.
As of v1.87d each custom field can hold up to 65535 characters.
To bypass ListMail you could use your own custom script then at the end use a
custom insert script for each list. In case of subscription to 2 lists, I would do the first one invisibly then forward the user to, for example:
http://example.com/mail/signup.php?fname=$custom_FN&lname=$custom_LN&email=$custom_EM
As long as you urlencode the names and trim the email address the user should be brought to the subscribe confirm or success page.
Regards