The two batteries do not "communicate" in any way, so while it's true that technically speaking you are twice as likely to suffer a fire (although it's 2X a VERY small number), it has nothing to do with the fact that you have twice as much battery material sitting in the garage. The chances of both vehicles spontaneously combusting at the same time (without some really obvious external cause) is infinitesimally small.
Sure, you do have twice as much battery material sitting in the garage, and one catching fire would almost certainly catch the other on fire, but the difference between one and two in terms of the risk it poses is pretty small. Even with one in there, if a fire did start, you are going to want to get out of there. The second one doesn't really add to the danger level. Besides, EV fires build relatively slowly so you should have enough warning to evacuate before anything reached the second vehicle anyway.
But let's talk about your charging situation.
I have a Model 3 and a Model Y as well. We have a single 30A EVSE that is plenty for us (full disclosure, I am getting an actual Tesla wall connector installed, but it's more due to the fact that we used to have two EVs with charge ports at the front of the vehicles so the current EVSE is in the back of the garage, while now that we have two Teslas, relocating the charger makes sense, and our utility has an EVSE rebate program that would be silly not to take advantage of). The point is, we almost never charge on the same day. We used to when we had two short range EVs that needed to be charged the same day, but nowadays, we each only charge every few days, and we easily coordinate which day each of us charges without impact. Unless you have extreme needs, I suspect that the amount of time both cars will be actively charging will be minimal (or at least you could intentionally minimize it if you wanted). And charging even at 30A is plenty sufficient to complete a charge in a single overnight session (we tend to plug in when we get down to 30% and charge to 80% and usually charging starts by 9:30 and completes by 3:30. If you are super worried, just set the charge rate down to 30A or even 24A.
But again, properly installed wiring should pose no more risk than your oven or clothes dryer or A/C unit. If done to code, there is plenty of safety margin to keep you safe.