Should serial triac dimmers work for resistive load?
Stacked dimmers are probably not going to behave the way you want. Household dimmers expect a sine wave input: they look at the zero crossing and fire a triac at some fraction of the cycle. When the second dimmer receives the chopped up sine wave, it might try and fire at the same time, or it might get confused by the non-sine input. It may even have undervoltage trouble, depending on how the low-voltage DC for the control circuitry is controlled.

Instead of two dimmers, here's a couple different things you could try to accomplish your goal:


Feed the hot plate from a variac. However, this could cause the hot plate to malfunction (again, feeding it a lower voltage might make the control circuitry unhappy), and large variacs with large power ratings are expensive, heavy, etc.
Control power to the hotplate with an SSR. Because the hot plate has a slow thermal response time, you could basically "PWM" the power to the hot plate with a very low frequency (1Hz or lower).
Break open the hotplate, and bypass the built-in temperature control (if possible, leave the internal temperature cutoff for safety), and control the hotplate with an SSR. This eliminates all uncertainty with the built-in control, and you have direct PWM control. Again, because it's a hot plate, you don't need high-speed PWM control, 0.2Hz or something will be fine. The nice thing about SSRs is that you can buy a cheap SSR with screw terminals, and you don't have to deal with any mains voltage other than wiring the thing properly.
The above suggestion might also work with a household dimmer. However, make sure the dimmer is rated for your hotplate wattage, and keep in mind that some household dimmers don't go close enough to zero (you might have the same problem).