同时进行AJAX请求和PHP会话

I'm having a peculiar problem with making simultaneous AJAX requests that modify a PHP object stored in the session. I'm having problems with race conditions but I don't understand why they're happening... only one AJAX call can have the session open at a time, so if they keep the session open while making modifications, shouldn't each call be able to see the changes made by the others? Here's a simplified version of the flow (A and B are two sets of parameters):I'm having a peculiar problem with making simul