In my opinion a 200km isn't as hard as 2x100km seems in your imagination.
Yes it's harder than 1x100km but you tend to find that your brain settles into the 200km end-point and 100km, which used to seem a long way, comes and goes fairly easily.
Pace yourself (Save any sprints for the final 5km not the first 5km)
Keep drinking
Keep snacking
Keep pedalling.
The final 20km will seem like 40km but, as other have said, that's the same on any ride.
You have all day.
Oh, I suppose most importantly, any comfort issues you have during 100km will NOT get any better during the next 100km.
Address any, even the smallest, comfort issues because discomfort follows an upward exponential curve with distance I've found.