Get a Plumber with a longer & much bigger snake & to also do the items below. Or, keep snaking the lines, while flushing repeatedly. Also snake from the roof vent side to clear any build-up that may have accumulated above the draining path. Many times Regular Tee fittings are used instead of Sanitary Tee's, this isn't correct or legal but are missed by Building Inspectors frequently.
Venting or Breathing sounds like your primary toilet problem. Toilets completely fill the drain pipe where sinks & showers do not. If a clog is in the venting it's putting "a finger on top of the straw" & defeating the plumbing's design.
Absolutely, get the Septic System assessed & pumped if necessary. Snake the leach field inlet as well, since something non-biodegradable may have been flushed by a guest. Waiting for a system to overflow onto the yard is not a good "time to clean" gauging method.
Now, when you have a problem, is the time to pull both toilets up. Snake & hose out their drain paths from both ends. Clear the toilet flange pipes to full diameter. Even replace shut-off valves, supply lines, fill valves & flappers. And, Install new toilet seals. Fix any & all problems or potential problems in one shot.