If both heaters are turned all the way up, and cycling on and off, then two things
could be happening:
1) The temperature of your tank is very high and they are switching on and off normally. A good glass thermometer will tell you the tank temperature for sure. Get one and see what it says.
2) Both heaters are functioning properly, but do not have enough output to raise the temp above the ambient basement temperature enough to see a noticeable raise in tank temperature (hence stuck at 75 - 76). Thet may have an internal cut-off circuit to keep them from burning themselves out prematurely. Again, a glass thermometer will tell you your tank temperature accurately enough that we can then fix this issue.
I would keep both heaters in the sump in order to avoid a thermal gradient in the basement that may have undesirable results. BTW, do you know what the flow rate through your sump is? It could be very slow, and therefore provide for a drop in temperature due to the basement ambient being so low. I recall you had an issue with the small bore PVC that was originally used for the return plumbing. Remember melev mentioned this, a long time ago?
Please try to gather all of the facts (temps, settings, etc..) that folks have been asking for (pictures won't hurt either, and I don't mean ones of the clowns or the rocks

) along with a different thermometer, just to confirm the numbers so we can fix this issue once and for all.
Oh, and no, the tank should not be kept at 75 F if you plan on hosting warm water species. That is just wrong.
Have you consulted your local reef club( you said you found one) about any of these things?