Doh! If you had 2 Nikon strobes, you could use the SC100 dual sync cord. I picked up a YS90DX recently and found out the hard way that while the two connectors (at the strobe side) look very similiar, they are not. Sea and Sea obviously did that on purpose. Good thing I had an appropriate sync cord...
I agree with Don that the Ikelite bulkheads are much better than the Nikonos style...but good luck getting Aquatica to switch them out. Anything is possible I suppose and I'm sure they have done it before...
Another cheap (but least reliable) method would be to just use a single sync cord and set the other YS90DX as a slave flash. The slave sensor in the YS90 is pretty good at "spotting" the flash from the other strobe.
So here's my Recs in order of reliability:
1) Two bulkheads on the housing and 2 sync cords (Nik to S&S)
2) Dual Sync cord (Nik to S&S)
3) One sync cord to one S&S
FWIW, I use a dual cord.
Good luck and happy shooting!
James