There are two things going on here.
First off, NVIS propagation. This is a function of the ionosphere and of your operating frequency, not the frequency that the antenna is "resonant" on. If you are working below the critical frequency, NVIS "works". If you are working above the critical frequency, nearby stations will be "in your skip" and won't hear you. Say you're working at 7MHz and the critical frequency is 6MHz, then your skip distance will be around 360km; if the critical frequency drops to 5MHz, then your skip distance increases to around 600km. Stations inside that radius generally won't hear you (unless you can work them line-of-sight or groundwave, of course).
Second, "NVIS antennas". An NVIS antenna is simply one that has a lot of high-angle radiation, no major high-angle nulls, and relatively little gain towards the horizon. Usually, these are horizontally-polarized antennas at low heights (compared to the operating wavelength). You don't need an NVIS antenna on either end, — it's just a few extra dB of gain in the direction of interest. Under some conditions you might need those few dB, but generally not.