Unfortunately you cannot have 2 singles from a 2 dubloon coin. The best you can have is a one dubloon coin, which costs nearly same/slightly more than the 2-dubloon coin, the last time I checked...
You can go to the Golden Dubloon, and eat the "green salad" (I think), which costs 1 dubloon. That should change your 2 dubloon coin to 1 dubloon.
I used to use it a lot when the 2-dubloon coin was worth 200 np less than the 1-dubloon coin. For 200 np only
- those were the days !
But remember not to have 1-dubloon coin with you at that time, otherwise they will take that one.