Interesting question. Flo sort of allows you to fake a wall, within limits.
Check out the attached screenshots. The part file is from SW 2011.
The two fluids must be of the same type, in this case both under the 'non-newtonian' category.
I used olive oil and polyethylene, since their viscosities are very different.
You must have an outlet BC with environment pressure.
I ended up running at 0.01 ft/sec inlet velocity, at 0.05 ft/sec or greater, SW crashes.
adding keywords, piston in a cylinder, syringe.
In the end, I ended up taking an analytical approach with a very advanced excel spreadsheet, and macro driven numerical solver, using thermo equations, etc. This worked pretty good, and allowed me to iterate the study a little quicker than with Flow. I did have to make some simplifying assumptions though