I'm a little confused about what exactly your question is...
Construct doesn't care what order the data is returned in.
As long as the triggering event On Completed with a tag is run, within that event, AJAX.lastdata will give you the data from that specific tag's request. It doesn't matter if it comes back before, after, or at the same time as another AJAX request.
Basically, you should never use AJAX.lastdata outside of an AJAX On Completed event. If you need to get it later, save it to an array or variable of your choosing.