While it is not perfect, you can use the following function as a document-wide function. The only problem is that the cursor is not currectly moved to the correct field when pasting text that spans multiple fields.
Acrobat 9: Advanced > Document Processing > Document JavaScripts
Acrobat 10: Tools > JavaScript > Document JavaScript
function tab_chain(prev_field_name, next_fields) {
// Move to next field if the current keystroke
// fills the field. Move to the previous field
// if the current keystroke empties the field.
// Pasted data that is too long for the current
// will be continued into the fields listed in
// the next_fields array.
var rest, prev, next, i;
event.change = event.change.toUpperCase();
rest = event.changeEx.toUpperCase();
var merged = AFMergeChange(event);
//console.println("Name: '" + event.target.name + "'");
//console.println("Merged: '" + merged + "'");
if (merged.length === event.target.charLimit) {
//console.println("Limit: " + event.target.charLimit);
i = 0;
prev = event.target;
next = getField(next_fields[i++]);
rest = rest.substr(event.change.length);
while (next !== null && rest.length > 0) {
//console.println("Rest: " + rest);
merged = rest.substr(0, next.charLimit);
rest = rest.substr(merged.length);
next.value = merged;
prev = next;
next = getField(next_fields[i++]);
}
// Update focus if previous pasted field is full.
if (next !== null && merged.length === prev.charLimit) {
next.setFocus();
}
}
else if (merged.length === 0) {
getField(prev_field_name).setFocus();
}
}
Then, call this function as a custom keystroke script under Properties > Format. As the first parameter you pass the previous field in the chain (or the field itself if it is the first). As the second parameter you pass a list of the following fields in the chain. For example if your fields are called IBAN1, IBAN2, ..., IBAN6:
Script for IBAN1: tab_chain("IBAN1", ["IBAN2", "IBAN3", "IBAN4", "IBAN5", "IBAN6"]);
Script for IBAN2: tab_chain("IBAN1", ["IBAN3", "IBAN4", "IBAN5", "IBAN6"]);
Script for IBAN3: tab_chain("IBAN2", ["IBAN4", "IBAN5", "IBAN6"]);
etc.