Nissan: Birkat Ha'ilanot on trees that blossom twice annually

Answer

With regard to olive trees, Peri Megadim (Joseph ben Meir Teomim, 18^th centery, Europe;  OC §226) states that we should not say birkat ha'ilanot on olive trees, since according to Rambam, there needs to also be a rejuvenation of the leaves. Trees that are not deciduous are considered evergreen; since there is no rejuvenation, we do not make the blessing on them. However, Responsa Or Letzion (Rabbi Ben Zion Abba Shaul 1924-1998, Yerushalaim; part III chap. 6, p. 67) states that it is possible to make the blessing on an olive tree, despite the fact that its flowers are smaller than those of other fruit trees. In practice, I have seen that the custom is to say birkat ha'ilanot on citrus trees, even though they are considered evergreens and there is no fundamental difference between them and olive trees. Apparently, we do not have to take into consideration the Peri Megadim's opinion.

About trees that blossom twice a year, I have never heard that trees blossom twice but only produce fruit once. I have not seen any discussion of this matter, either. However, I believe this can be compared to the discussion about making the blessing over trees of orlah status (see Yabiya Omer Part V, OC §20). Some say that the blessing should not be made on such trees, since we cannot derive benefit from the fruits. Others say that the blessing is being made on the species in general, from which people certainly can benefit from, since most are not orlah. This is similar to the case you described, since as you wrote, the tree will bear fruit but not from the current blossoms (I hope I understood you correctly).

In practice, it seems to me that if you do not have a different tree, you can certainly say birkat ha'ilanot on the trees you mentioned above. That is: it will be possible to benefit from the fruits of this tree. It doesn't seem that you would need to know which flower will bloom into a fruit.