Running two app engines at the same time that use/update the same table

I hope this makes sense:

I have two different app engines that update the same table. If I run both app engines at the same time, is the processing/updating compartmentalized to each app engine? Or will the updates from one affect the other's process?

For example:

App engine AppEngA inserts a bunch of rows to table PS_MyTableA. The table has a field to indicate whether the row was processed, it gets inserted as 'U'. When the app engine is finished it updates the field to indicate it's been processed: it updates all rows with SQL that says if the field = 'U', set it to 'P'. There really isn't any field (such as process id) to identify what process inserted the rows.