Have you verified they are not the same circuit? Different rooms can share a circuit, it depends on the layout and electrical runs.
And what else is on the circuit with the heater? The combinations could push it past the trip point.
You didn't answer the Q about "how many are tripping?". Two different circuits tripping, with two heaters?
Exactly what type of space heater? Again, most heaters will draw the same watts/amps on "LOW" as they do on "HIGH" - it will just run for less time.
Unsafe is overloading a circuit. While it is possible your circuit breakers are over sensitive, I'd rule out an overload first.
In the OP you said they "don't immediately trip." That would only happen if you had a direct short. A typical overload condition can take minutes, even hours to trip.
-ERD50
UNLESS you've verified which breaker EVERY outlet is on - plug a light into the outlet then turn the breaker off and back on, phones or yelling through the house to coordinate - you don't KNOW that the outlets in different rooms are on different breakers - or that the breaker hasn't been multi-tapped (NOT to code) and the breaker for the dishwasher is also feeding the outlets in one or more rooms. This house (almost 50 years old) has had a lot of "need to add..." from previous owners and I spent several days mapping the lights and outlets to their respective breakers. Here are a couple of bad examples of what I found:
Garage lights, garage door openers, garage inside wall outlet, outside lights over driveway, laundry room lights & outlets (including freezer), screened porch, corner floodlight over lower drive
Disposal, family room and kitchen lights, master bedroom closet, outlet by master bedroom closet
If a heater was plugged into that "outlet by master bedroom closet" and someone turned on the garbage disposal, you'd probably be very close to the trip level of that breaker. Even more likely to trip if the heater is in the laundry room, the freezer is in its defrost cycle and someone opens or closes a garage door.
Do you KNOW how things are wired?