When is it not a good idea to use both attacks in clan war?
In most cases those are just assumptions whether you are going to win or to lose. But you can browse through your enemies and compare your strengths: If your enemy is roughly equal and most of the opponent clan members have used their attacks already while your clan has most of them left, then you are able to say that it is likely that you will win the clan war.
And reasons why some clan members would not attack in this case? Well, if you will probably win the clan war and the remaining opponents all have bulwarks for which you would need plenty of troops while the possible loot is nearby zero, then it's wise not to attack those enemies.
So you always have to weigh between the possibility of winning the war and the profitability of attacking an enemy concerning the loot.
Almost never
For starters, not using attacks in a clan war is disrespectful to your fellow clanmates who actually want to participate in the war. Clan wars can be fairly unpredictable and it is nearly impossible to determine how a war will end without at least getting half of your available attacks in. However, if a good number of your members refuse to attack, than that will almost certainly decide the fate of the war.
When you lose a clan war, every member who participated in the war will receive 40% of the war loot that they would have otherwise gotten if they had won. If you had successful attacks against bases that allow you to earn 500k gold/elixir and 2k dark elixir, then losing the war will still net you 200,000 gold/elixir and 800 dark elixir. If members do not attack because they're convinced that you are going to lose the war, then 40% of 0 gold/elixir/de is still 0. So, that's loot that they're missing out on, especially if there's a particular resource that they're trying to accumulate. Even if it seems clear that your clan will lose the war, scoring 1 star against bases can still be done fairly cheaply with, say, giants + barch + healing spells.
Speaking personally, though, I always go full out because I love to use war attacks to smash bases. If it's clear that there's going to be a losing effort, I'll do one of two things:
- attack the most difficult base that I think I can 3 star if I want to practice a particular attack strategy
- attack the most difficult base I can that I think I can score at least 1 star on.
I usually go with option 2, which tends to involve attacking bases with town halls higher than my own that have some exploitable base design with troop/spell compositions that I wouldn't normally use. But it's immensely satisfying to score 1 or 2 stars (and thus, earn the higher loot bonus associated with the base) on a base that's 1 or 2 levels higher than me. Do I net more resources than I spend? Probably not, but I don't particularly care - this is Clash of Clans. I'm here to build up my base while smashing other bases. If I cared only about building up my base, I'd play SimCity.
In my Clan wars I normally save my 2nd attack encase I need to attack last minute. But sometimes it is mathematically impossible to win.
Like if I am the only one with an attack left and they are up by 3 stars and I can only gain 2 stars by attacking. So in the end results I would have wasted my Elixir and we would lose the war.