How can i do something like multi threathing in arduino?
You will not have full parallel execution in an Arduino (only one CPU).


You might do multi-threading by implementing a small OS (e.g. FreeRTOS) 
Or write your own task scheduler. 
Or even just do state machine looping forever checking if push or pop has to be executed.
Last and preferred solution: I would consider running the normal Push task always and execute the Pop task (function in fact) using interrupts.