Assuming I understand correctly...say you've created a 2-bar sequence in the step sequencer and want to extend it to 4 bars in the step sequencer, with the second 2 bars being automatically copied from the first 2 bars...right? Then this "xpands" on what Sanderxpander said:
1. Go to Track view and drag the MIDI file to the right to add 2 more bars.
2. Right-click on it and choose Bounce to Clip.
3. Right-click and choose Convert MIDI Clip to Step Sequencer.
Now when you double-click on the clip to open the step sequencer, you'll have your 4 bars with the last 2 duplicating the first 2.