How can I force sequential execution on 2 simultaneous requests?

I've got an input form with 2 inputs.
Each input value (not the whole form) is sent to the server immediately after it's set.
I can pretty much make both inputs submit simultaneously.
I fill in the text box and without leaving (blur) I click a checkbox.

The server gets the second request before it can process the first.
The second needs an ID that is generated during the processing of the first.
The second is so quick that it precedes the generation of the ID so it thinks it's the first and...

How can I force sequential execution on 2 simultaneous requests? WordPress ...