Water Heaters - Additional water heater

I am installing a new whirlpool bath. The house currently has a 50 gal water heater. rather than install an optional (read overpriced) in line heater at the tub that requires a dedicated new circuit, I am thinking about increasing the size of the water heater. I could buy an 80 gallon ($300 DIY install), but my son asked an odd question that I couldn't answer.
Can water heaters be "daisy chained" either series or parallel? I have access to a new 40 gallon electric heater at a good price (free). My thought was that both could be put on timers to ease the increase in power consumption.

The "optional in-line heater at the tub" is probably a heater on the tub itself and not on the fill line. The purpose of the tub heater is NOT to heat the tub water but to keep it from cooling off as you enjoy a nice long soak. The tub heater's thermostat is most likely set no higher than 105 degrees F.

As for your question; yes, you could plumb both heaters in series and it would give you more hot water to draw upon but the first heater in the line would do the lion's share of the heating with the second heater only coming on as the output from the first heater dropped below the thermostat setting of the second heater.

Connecting the heaters in parallel would also give you a greater initial amount of water and both heaters would probably "see" the same heater usage.

Of course you would need to add a new dedicated electrical line to the new (additional) heater.

Depending on several variables, time clocks on the water heaters may, or may not, result in significant electrical savings.