Dash Rendar (BotF, 0426) has following text:

" Action: During an engagement, spend 1 resource to have this unit gain a {unit damage} icon until the end of the engagement. Any friendly player may initiate this effect."

Since each action is separate and has to be paid for independently, am I correct that I cannot focus a two-resource producing card by placing two focus tokens to trigger this action twice? Instead I have to focus a resource producing card for one resource to trigger it once, then after my opponent has a chance to take an action, I have to focus another resource producing card for one resource to trigger it the second time. Is that right (that there is nothing like Mana pool in this game)?

That is correct.

ok got one question on this, The light side wins the edge battle, I open an action window and spend a resource to give render a blaster, if mu opponent doesn't open an action window can I open a second one for a second blaster before the attack ?

That is correct. In each action window, starting with the active player, players execute actions until both have passed.

... until both have passed consecutively. You can come back in after you pass if your opponent does something.