Currently, you cannot have two dijit.form.SimpleTextarea fields in the same page. Both fields (field 'a' and field 'b') will end up being submitted as a single param ('a') and the text entered in both will be merged under parameter 'a'
field definitions to reproduce:
add field string -class ~.domain.Test -fieldName a -sizeMax 70
add field string -class ~.domain.Test -fieldName b -sizeMax 70
Firebug shows that a is submitted twice with different values (once the value from textarea 'a' and once from 'b'):
This results in field 'b' not being populated in AbstractPerson and field 'a' containing: 'TestA TestB'