How to determine tankless water heater requirements?
With a tankless water heater it's all about how many degrees the heater can raise a certain quantity of water.  In your situation, you will need a heater to raise the temperature of about 5 gallons per minute (two showers running at once or one shower and another hot water tap running).  If you live in the north, and the water out of the tap is be 40 degrees in the winter, then you will need the heater to raise that 5 gallons 70+ degrees.  If you live in Florida, you may only need it to raise 5 gpm 45 degrees.  Information about a water heater you are considering purchasing will include a chart that specifies temerature gain at various flow rates.  Choose the one that best meets your demands.