You can mount another jet without any supply issues, the pump is up to the task.
With jet selection I use a ratio of water to fuel. You will have no issues running 2 jets unless they are way over sized for needs = flood the combustion chamber with too much cooling. I have ran two jets similar to as what you described. I think this is a fantastic way to make the most of water injection. Typically jet location was a trade off, closer to the IC exit tank the better for intake air charge cooling, the closer to the combustion chamber the better knock suppression. With two jets you can tackle both of these.
Heres a link to help with jet selection.
http://www.srtforums.com/forums/showthread.php?t=306782
Now for which jet where, by using the jet adapters you can easily experiment with jet locations or easily swap jets around while doing a dyno or road tune. I found out that my best results were with two jets, the bigger of the two was located near the TB and the smaller was nearer the IC.......one jet was 0.7 and the other was 0.9 using the Aquamist race pump.
With my two jet system I slightly violated the 10%-15% rule and had about 20% - 25% of water.......I had some fuel tuning going on so it worked for me.
If I were you I would get another jet adapter fitting located near the IC exit tank and then add in the second jet and use the "Y" fitting connecting the two to the HSV. Do some runs and then swap jets around and do some additional runs to see what works best for you.
Contact me if you need additional hardware or jets.
