A GT double takes more effort... that much is obvious. Is it more difficult to win? I actually think the opposite. When you are the strongest over 3 weeks, you are almost certain to be in the leaders jersey after day 21. To win a 1 day race however, is a lot less evident. Indurain won 5 TDF's back to back. Armstrong won 7 back to back. Froome won 4 (and should have been 5) back to back. When was the last time a classics rider won RVV 5 times back to back, or 7 times Roubaix back to back? Closest we have is Pogacar in Lombardia for the moment. To win a 260k 1 day race, it has to happen on that day. If you aren't by far the strongest, a lot can happen. In a GT, as long as you don't crash or get sick, you have 21 days to make up a deficit or strike the winning blow.
OG only happen once every 4 years. So winning an OG double + WCC double, you only have 2 or 3 chances to do that in your career. And to win on those 4 days in the same year, you need to be the absolute best on those 4 days, because you won't win those 4 days by sheer luck. I honestly don't see anyone in the current peloton, Pogacar included, who could do it, other than Evenepoel. Beating big specialists at the end of a GT in a 25k mountainous TT is not the same as in a standalone TT over 45k with medium hills at best. So if he were to pull it off, it would most certainly be more unique and i'm sure time will prove it.
Both Roglic and Vingegaard could have won 2 GT's in one year, last year. Roglic came close in 2020 as well.
He 'll want to follow in the footsteps of his big idol. Van Wilder