This is dependent on a lot of factors we don't know, such as the average temp of the room,
Actually we do have this information, it was given in the OP
open/closed tops, etc etc.
Given the OPs concern this information is irrelevant.
Running undersized heaters is a good thing, but I wouldn't claim it to be a cut and dry fail safe.
See my previous post as to the explanation as to WHY I gave this advice. I don't really see how I can make it any simpler.
Though I will agree with you on your point regarding a separate probe based thermal controller, and that would/could be another route should the OP choose to buy only one 300w heater in this case. But unless they are willing to buy a thermal controller ALONG WITH the 300w heater, IMO the better course of action are 2 heaters slightly undersized as opposed to 1 over sized heater. At least if it were I and my 20+ years of experience (not all planted tank based) in this hobby needing to find a solution such as the OP has queried about.
As for me personally I now have the heater on my main tank tied into my Apex controller, and as for the heater in my bedroom tank... For now I'll trust the BRAND NEW Jager I installed 2 weeks ago, that tank isn't as heavily invested in that I'll loose sleep with worry.
Okay let my give you the run down of my tank set up, 40 gallon with an aqua clear 70 with a Hydor koralia 240gph, currently with the eBay 200 watt heater the tank is heavily planted idk if the matters or not. As for heaters suggestion the dual heater with a thermostat sounds like a great idea but I was also looking into the fluval E series, the M series also made by fluval, jager heaters and the Finnex Hang-On Electronic Controller Aquarium Heater, Titanium Tube as reliable heaters to go with. The temp of the room in the winter time is 60-75F
Jagers have an excellent reputation, I was running Marineland heaters and both crapped out after a year so I suggest staying away from those