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I don't see the benefit or the practical application of your first option. If you factor in installation costs your best bet is either ! large tank or turning up the temp on a smaller tank and installing a tempering valve to return the water temp to a safe level.
Yes you can pipe 2 tanks together.
If you now have a tank that's too small installing a tempering valve and turning up the tank temp is the cheapest option.
Of course if you currently have an electric water heater then you could use a gas tankless with the electric water heater providing some storage and extra capacity but in this case your best bet if you really want to go tankless is to just get rid of your electric tank and put in a tankless with adequate capacity for your needs
You're better off to install 2 tank type heaters and size them for the demand of that area. Tankless heaters are over hyped and all things considered (installation, fuel cost and maintenance) they cost more than tank type heaters. There is nothing to be gained by connecting the 2 tanks