Unless you're opting for tankless primarily to gain back a few square feet of floor area moving to an underpowered electric tankless is usually a bad idea. For the money you can buy more efficiency using other methods, and at very low money the standby loss of the tank while your away can be brought to zero by either turning off the breaker (or installing a switch to throw) when you leave. (Turning off both power and the water inputs to any hot water heater is a good idea for any place that sees only intermittent use.)
If you have to upgrade the electrical panel or run a dedicated 75A/240F line to serve the 18kw water heater (which is likely) the payback on reduced energy use would be measured in centuries, and that's assuming you DIDN'T have the option of turning off power to the tank heater when you leave.
But assuming you're going ahead with the project, the simple math on how much power it takes to run two simultaneous 1.5 gpm showers with 65F incoming water:
Shower output is about 105F, so you're looking at a (105-65=) 40F rise.
1.5 x 2= 3 gallons per minute. Times 60 minutes is 180 gallons/hr.. Time 8.34 lbs/gallon is 1500 lbs/hr.
1500lbs/hr x 40F= 60,000BTU/hr
Divide 60,000BTU/hr by 3412 BTU/kilowatt-hr and you get 17.6 kilowatts.
So you have essentially zero margin on being able to run two simultaneous low flow showers. During the coldest winter months or a cold snap when in coming water temps drop to 60F it wouldn't keep up, but you'd always have enough flow for one.
If somebody decided to start a load of laundry or dishes when someone is in the shower, don't count on it keeping up with those intermittent but high-rate flows either. Scheduling the other moderate to high flow hot water use as to not conflict with showering isn't difficult, but it probably would be necessary. (Been there, done that- I lived with an undersized gas-fired tankless for more than a dozen years in cold-water country.) This might be OK for a weekender spot, but most people would rather not live that way.