I hit my ult once to let ana know im ready, then hit the emote once again to let ana know to boost me. Most anas are pretty perceptive that if you emote your ult at 100% twice you want de boost my brudduh
You shouldn't have to. Nano is a reactive ult. The Ana player should use it when an ally is in a position to do lots of damage - ulting Genji, McCree, Rein, etc