How to synchronize Ajax requests in a form?

In my form I have 2 input select ‘OWNER_SELECT’ and ‘ITEM_SELECT’
(owner has many items)

an observe_field on OWNER_SELECT modify the ITEM_SELECT options and set
to selected value

an observe_form post a request search all data and refresh the list

so I have 2 post requests, is there any way to delay the form post
request until the first post (replace the ITEM_SELECT html) is finished
?

kad