I'm not sure about your question, but draining both the sink & the shower
into the shower sump, and tying the sump output through the anti-siphon loop to the existing thru hull
I have an extreme adversion to thru hulls and use as little of them as possible. My previous boat had seperate head
sink that drained directly to a thru hull
, and shower sump that had it's own thru hull
I wouldn't tee the output of the shower sump to the head sink drain, unless you didn't mind your shower water
backing up into the sink.