To install 2 elec water heaters, do I need to put them in series or parallel

It really depends who you talk to. But, both installations are perfectly OK.

I'm assuming you mean water and not electric:
I find if you hook them up in series the first does most of the work. In parallel they'll both wear out at the same speed.

If you plan to run them in series, or parallel, I would only suggest placing shut off valves so that you could bypass each water heater while leaving the other water heater in use. This way if one of the water heaters has a problem, you can still use the other w/h while repairs or replacement are taking place. Please let us know what you think.

to install 2 elec water heaters, do I need to put them in series or parrallel

Your choice!
In series the first heater heats the water and sends it to heater number two that maintains the heat and supplies the draw. The volume is regulated by the size of a single heater.
In parallel both heater supplies and outlets are connected together thus doubling the volume.
As I said, your choice. Good luck, tom

We also prefer in-series installation. First heater pre-heats water to 110F and second heaeter brings tepmerature to 125F.

If the heaters are gas heaters than make sure you have 4" vent to vent them...

I prefer parallel with separate shutoffs and check valves for each tank, because this gives you the option of isolating one of the tanks from the system should it fail.

Henrib,
And there you have two opinions from working plumbers. As I said, Your choice! Good luck in whatever you choose. Tom