In a gusher-shower you can roughly double the showering performance with a
drainwater heat recovery heat exchanger, provided you have sufficient headroom on a vertical drain downstream of the shower for 4" x 48" tall (or larger) unit:
(Best of all, it doesn't increase your power use, or burn any gas.)
A gas-fired unit in parallel with an electric unit is difficult to control. The major high-end manufacturers of gas tankless heaters have special controllers to be able to parallel them with good results, but it's damn-near impossible to make heterogenous tankless systems "play nice" together, and would likely require a custom set of controls and a man-year of engineering time to really get it down.
With cold-water only (so that the water heater isn't intentionally restricting it, bucket-measure the true gpm it's drawing- unless you have unusually high water pressure you're unlikely to hit the gpm-ratings of the shower heads, but it'll usually be more than half the rated flow. Shower head rating are a max-number, rated at 80lbs pressure. Most homes have water pressure half that or less, and typical 2.5gpm shower head it typically running in the 1.6-1.8gpm range, or about 2/3 of the rated flow. If your true flow is in the 8 gpm you can probably get there with a 4" x 48" to 4" x 60" drainwater heat exchanger.
The
manufacturer will sell direct at a retail type pricing- if the US distributors & retailers don't stock the larger sizes.
Home Depot carries some, and can be shipped to a store near you, but I don't see any thing bigger than a 4 x 48" or 3" x 72" listed. EFI's warehouse is in WI- you can open an account with them over the phone with a cc# to get their wholesale price on whichever models they are carrying.
There are
others in the
biz, but you need something with an efficiency
better than 50% as tested per Natural Resources Canada, but that also doesn't unduly restrict flow. Something in the 60% range like a 4 x 60" to 4 x 72" would be better, since they're tested at 2.5gpm, and you'll be running more that 2x that flow. A 60% @ 2.5gpm unit would still be delivering over 50% @ 10 gpm.
If that doesn't cut it, a monster-sized tank is your best bet.