It won't heat as good, if at all. The screens are resistance to the circuit. The more screens less resistance, less resistance less heat. "A watt is a watt, no matter what." Ohm's law.
I don't know about much of that. First off, that's now how resistance works. Yes, two screens in parallel is more resistors but since they are in parallel (not series) the total resistance goes DOWN, not up. Two identical loads in parallel cut the resistance in half in fact. Normally causing more total current. In a perfect world, twice as much current so the total power also doubles. Just like turning additional room lights gives more light and a bigger electric bill without changing the light from each bulb.
But this is not a perfect world. Just like the lights dim when the washing machine starts (high load current), there are 'line losses' here. Very large ones. Normally MFLB is about 15 Amps at about 3/4 of a Volt. The battery is about 1 1/4 Volt so almost as much power goes to heating up the battery (internal resistance losses) as heating the screen. Not good for battery charge life or longevity. A 'diminishing returns' game.
Normally lower resistance means more current which means more power developed.
My guess is multiple screens won't get hot enough as individuals. And even if they do, battery life will take a serious beating. I bet patients with the factory configuration works better, bowl/trench size was no doubt tested a lot.
By all means try, but my guess is the results will be 'sub wonderful'.
OF