IIRC, the aftermarket chinese ink for latex faded pretty quick and caused a lot of extra head wear and replacements.
This was 5ish years ago.... Maybe it's got better? But something to think about... Even if the ink is cheaper, if it causes you to replace your heads more often, it probably negates the savings.
If you have 2 printers I'm assuming they're both mostly busy.... In which case you may not need to look into ink savings, but more in jacking up your prices?
Ink cost is pretty negligible compared to other things, if you hacked up your prices an addition 15-25 cents a sqft, you'd cover any savings in going to aftermarket ink, and I bet I wouldn't effect your bottom line at all. 25 cents a sqft more makes a 4x8 sign be less than $5 extra.