I could see it is slightly manpower negatives, but only slightly. A huge positive for pilots is that are current and qualed on two aircraft types. Easier to get a contract job if the economy takes another dump.
I know the systems are vastly different but in the case of these two aircraft I would say it doesn't matter. Boeing doesn't really require you know squat to be rated on the 777 and I would bet the 787 is the same or easier. so you don't have to know nada about two airplanes vice one. I believe Boeing has approval for a common type with 2-5 days differences training. Can't remember if it was 2 or 5.
757/767 was much more difficult IMHO. They land much different, especially banging the tale on the 763 vs pounding the nose gear into the ground on the 752. And don't get me started on smoke and avionics cooling. I think I had it all memorised correctly for all variants, once, for 5 minutes. Luckily that was the one time I was asked about it on my first type rating oral. LOL. I have done the course three times during my yo yo up and down UALs financial debacles.