Are two binoculars better than one? Yes, of course 
Now I saw this in your first post, and to me, it's the only line that matters:
In English I would call this a no-brainer. The 10x42 SF are significantly better than either of the binoculars you own. For me, I would move my 2 bino collection to the 10x42 SF and the 8x40 SFL without hesitation. The Viper just can't play in this league, that will be readily apparent once you have the SF's. And the SF's are so ridiculously high in price, it's very rare to get a chance to buy them for less money.
I would consider the 8x40's and the 10x42 to be the perfect combination. Low power and high power, with an extra edge of portabliiyy and lightness for the 8x40's. DO NOT sell the 8x40 SFL to buy 8x30 SFL. Personally I did not like that bino at all, it's so small and light than I can't hold it steady. 30mm is small aperture. Those are a portable specialist for backpacking or some other use where you need to carry a tiny optic IMO.
Just for perspective, my primary two birding binos are 7x42 EDG for low power and 8x42 SF for high. So it's not unusual to have two binoculars that are close to the same size and magnfication.
Now I saw this in your first post, and to me, it's the only line that matters:
Now I'm in a situation where I've been offered to buy a pair of SF 10x42s at an incredible price.
In English I would call this a no-brainer. The 10x42 SF are significantly better than either of the binoculars you own. For me, I would move my 2 bino collection to the 10x42 SF and the 8x40 SFL without hesitation. The Viper just can't play in this league, that will be readily apparent once you have the SF's. And the SF's are so ridiculously high in price, it's very rare to get a chance to buy them for less money.
I would consider the 8x40's and the 10x42 to be the perfect combination. Low power and high power, with an extra edge of portabliiyy and lightness for the 8x40's. DO NOT sell the 8x40 SFL to buy 8x30 SFL. Personally I did not like that bino at all, it's so small and light than I can't hold it steady. 30mm is small aperture. Those are a portable specialist for backpacking or some other use where you need to carry a tiny optic IMO.
Just for perspective, my primary two birding binos are 7x42 EDG for low power and 8x42 SF for high. So it's not unusual to have two binoculars that are close to the same size and magnfication.