Delete the second drummer region
Then as was said earlier.....
Now extend the first Drummer region.
To do this... Move mouse to righthand BOTTOM corner of region, left click and drag... (Not loop which is done by clicking and dragging the pointer from the TOP righthand corner.. but extend) to the length you want engaging the drummer follow mode for that region..... and selecting the other settings you want for that Drummer region in the Drummer interface.
The single region will then look like this... (In this example I extended the drummer region to 24 bars.....)
Note: Because of how Drummer works.. if you are using fills you might want to only extend in multiple of 8 bars..... so 8, 16, 24, 32 bars in length and so on.... so the fills occur in the normal places.. ie at the end of every 4 and / or 8 bars.....