In conjunction with Singapore's 40th birthday, a local health & nutrition store is having the following promotion:

40% off the 2nd bottle
25% off the 1st bottle*
* - only for one item

This would suggest that:

If you buy 1 bottle, you get 25% off it.
If you buy 2 bottles, you get 40% off the 2nd bottle, but pay full price for the 1st; i.e. if 1 bottle costs $100 & you buy 2 bottles, you'll pay a total of $100 + $(100-40) = $160, hence an effective discount of 40/200 = 20% per bottle.

Then wouldn't it make more sense to buy each bottle separately under the 25% discount?

Hmmm.