I would use EVT_TEXT together with a wx.Timer. The idea here is to monitor how long it's been since the user last typed something in that second control. So you'll have a start value of zero (self.start_value) and each time EVT_TEXT fires, you'll reset that value and restart the timer. When the EVT_TIMER event gets fired, it will increment the start value. You'll have to put an "if" statement in timer's event handler that checks if the start value is greater than X seconds. If it is, then do the check to see if the first input if less then the second and act accordingly. Make sure you stop the timer if it is as a running timer object can hang the app when you go to close it.
Here's a link on timers: http://www.blog.pythonlibrary.org/2009/08/25/wxpython-using-wx-timers/