Hmm, well actually, it would be hilarious considering since Nami and Usopp kinda act like brother and sister, both are scaredy cats, weak as shit. And to vs two siblings who are very strong.... amongst their other characteristics... I don't imagine them winning though.
Well, try to reread chapter...when Usopp made look like that was Nami insulting Ulti, Nami for a moment was angry, but than said that iit's ok, case battle already started, and then said to Usopp that now they have to run.
Ok, firstful I doubt that UsoNa running away, actually I do thing that they have a real plan or something like that. I mean archer and mage classes always run away from warrior who chase em, till in some moment thing turn different and warrior who was chasing em, became a bait. So here I actually feel that something simmilar will happen. I mean Ulti is smart as Luffy, P1 could be smarter, but he depends on his power, while Nami and Usopp are smart. So I ask you, could you imagine than Nami and Usopp could make a trap for siblings? I believe answer will be, like Yes. So in those terms only problem that we got here is the way how sibling could be knocked out.
But again this will be fun battle, I mean it's already hillarious, that weak "brother and sister" of SH crew was the first one who started the real battle, and already running, which reminds me Usopp first battle againts Chew(fishman guy), which was hillarious. Now this battle could became one of the greatest and funniest battles in One Piece, case Oda really can make this battle like that. And again this is not ass pull, that we saw in 990 chapter, when Robin smiling was dealing with Younko. This battle even if from the begining feels like impossible, now it really could work, even without ass pull plot convinience.
But yeah, problem is damage...but again Usopp created Nami's original weapon, and he was the one who made hew new weapon, so technically he should knew everything about it. That means that both of Nami and Usopp could come to some strategy that will never could work, if they being alone, but together it should work. Now what I mean? Nami's wapon is nature power, and Usopp weapon is also Nature power, but different. So yeah, let's see how this will end ha- ha.