If you have a vertical drain pipe downstream of the shower(s) you can roughly double the showering output with a 3" x 60" or 4" x 48" drainwater heat recovery heat exchanger:
They won't do squat for tub-fills since the drain & HW have to both run at the same time to get the benefit, but they're quite good for gaining showering capacity. Better still, as a system a tankless + DWHR unit together have an "apparent efficiency" well over 100%, since it's taking back heat that was otherwise quite literally running down the drain. It uses no fuel, it's a passive device, but it definitely works, at far lower cost than adding or replacing a tankless.
Key to getting the full benefit, the DWHR unit has to feed the cold side of both the tankless AND the shower. That's often most easily achieved by having it feed the entire cold water distribution to the house. The output temp of the DWHR in summer may hit the mid-70s F when the shower(s) are running, but otherwise has no effect on the cold water side. If you have to choose either feeding the cold side of the shower or cold side of the tankless, not both, feed the tankless. That reduces the recovery efficiency of a ~50% unit to about 40%, but that's still enough to get the showering performance you're looking for.