Tankless Experiment ?
You need to make sure you size your gas tankless correctly, otherwise you will not be able to run simultaneous applications, like 2 showers going at the same time. All gas tankless water heater manufacturers advertize flow rates in the 6 -8 gallon per minute range. Read the small print - these flow rates are based on a 35 degree rise, so in your case with inlet water temperatures of 45 degrees, anything smaller than a 180,000 BTU model will not work for you. If you are told 140,000 BTU is more than enough, then the salesman or installer is not doing you justice. Here is why... If oyu buy a gas tankless rated at 140,000 BTU, Energy Star Rated, the Energy Efficiency being that of 82% it means that the 140,000 max BTU is really that of 114,800. If you want to figure out what this 114,800 represents as it pertains to Temperature rise and flow rates, here is the formula. Convert the 114,800 to KW by dividing 114,800 / 3414 (1KW=3414 BTU) = 33KW. Now that you know the KW you can figure out the temperature rise of the unit. Take 33kw and convert to watts (1kw=1,000 watts) so 33KW = 33,000 watts, then divide by 147 (this is the wattage needed to raise 1 galon of water by 10 degrees in an instantaneous basis) ( its actually 1470, but divide by 147 so you won't need to carry the decimal).
33kw x 1,000=33,000 watts / 147 = 224 (this is the tamperature rise at a 1 gallon per minute flow rate) Now take the 224 and divide by the actual flow rate you need, say 2 showers simultaneously. Keep in mind that all US made shower heads are rated at 2.5 GPM at 80 psi, with normal residential water pressures in the 55-60 psi they really only put out about 2 gpm. So 2 showers running at the same time with an inlet water temperature of 45 degrees as you mentioned will only come out at 101 degrees, (244 temp rise at 1 gpm / 4 gallons of flow = 56 temp rise + 45 inlet = 101. Get my point, the advertized 7 to 8 gallon per minute flow rates are not accurate. In your case you need at least a 65-70 degree rise so your outlet hot water is at least in the range of 110-115 degrees and for this you need to have at minimum a unit rated at 180,000 which is only going to supply a max BTU output of 147,600, enough to run 4.8-5 gallons per minute or 2 showers at the same time.