I have read Donald X. in at least two places say that a common beginner mistake is to swindle someone's 3-cost, be too scared to give them a swindler, and so give them a silver instead. This surprises me a lot that this is a mistake because I would really rather my opponent have one swindler and a silver than have two swindlers - they might collide, but they might not! And agh! Two swindlers! But I mean, it's Donald X. Maybe I am overrating swindler? Is swindler/swindler or swindler/silver the stronger opening?
As usual "it depends on the kingdom".
I think that usually swindler swindler is the stronger open. Especially if there are useless cards at 3 and/or 4, or if your opponent opens with a 5 or 2 cost card. If you can swindle silvers into loans or moneylenders into potions, swindler just becomes crushing. Also, the chance of your opponent hitting your only swindler with his swindler does come up, and that's a bad place to be in if you don't have another.
On the other hand, if its a money board where vault+BM is the best thing going (other than swindler), I'd probably only open one. Hitting their vaults is helpful, but in this case hitting their coppers is worse than normal, and they will have a number of gold which are also useless to hit.
I think its rare I would swindle my opponent's silver and give them a swindler. But it could happen, say if they already have two, and there are no villages. Of course there also has to be no better option, as chancellor would be more fun.