It would be very hard to play 2 then 1, but they're good enough to play both, so start with 1.
I think I put 20 hours into 1, maybe 16, and about 40 into 2. I didn't agree with those saying it was too hard with 1 person. Some of the nightmare levels were rough to 5 skull but nothing was hard to beat. Endless was underwhelming though.