Flushing issue with toilet
Even though you snaked the line near the toilet, it sounds like the line is still blocked or slow draining such that standing water sits in the line above the point of connection to the vent.  Vent lines for toilets are often connected as shown:



Standing water above the reducing wye where the vent connects will cause water to be pulled from the toilet bowl as water flows down the line (such as when the other connected toilet is flushed).  A flood when you flush both toilets indicates the sewer line is slow draining and cannot handle that much flow.  Possibly the vent is plugged.  More likely there is an obstruction such as tree roots infiltrating the line.  If you have a septic system then it is time to pull the inspection lids, especially the one at the inlet.  Check for obstructions such soap caked near the intake baffle.