Christian,
You are correct, the Sea & Sea NX-80 has only one strobe bulkhead connection... This is the only flaw I have found with this superb housing...
There are several options however, for utilizing dual strobes:
The easiest option would be to attach a Sea&Sea Dual Sync Connector (part #03450), which is compatible with all Sea&Sea "N" series or Duo Strobes... cost USD $235+/-.
Another option would be to consider using only strobes that DON'T have attached sync cords. This basically removes the YS-60 strobe, leaving you with the YS-30, YS-90, YS-120, YS-300 & YS-350. Purchasing only one single sync cord (which usually comes with any YS strobe purchase anyway) and one 5-pin Dual Sync Cord (part #03470...cost USD $155 +/-) would allow you to run any single or dual strobe setup (except the YS-60).
I have both connection types and both work very well. I am partial to running the YS-60/YS-120 dual setup - as this can give a more 3-D effect to my photos - so the Dual Sync Connector was a must for me.
One last option, although I wouldn't recommend it, would be to "retrofit" the accessory bulkhead connector located on the bottom of the housing... This connector is pre-wired for a shutter activated focus light. Although I know of nobody who uses this feature, I believe using this connector as a strobe connection would get in the way of the fisheye dome port.
Hope this helps.
/Brad
P.S. I forgot to add.... each of these options allows for full TTL strobe connections... both in single and dual strobe setups..