This dam idea for flexo is not a very good one.
In offset, it can be done with dividers to run as many as 3-4 spot colors in a fountain. But in between the colors, dividers, lubrication has to introduced to prevent the burning up of the non printing area of the ink train. this is usually accomplised by the use of an open pocket compound which serves as the lubricant. An open pocket compound can either be a grease, vaseline type substance or a very stable medium to high body varnish.
Back to flexo, trying to spilt an ink reservoir would be futile, as the low water like viscosity of the ink would spread through the unit. The risk you would run with the dam, is that the anilox roller would likely become damaged because on no lubricating fluid to sustain the non image area of the print unit. These reservoirs are also more like troughs rather than a more shallowed offset ink fountain.
D Ink Man