A horse can easily hold its rider and a second man. The Romans did this rather often, not only with their velites, but also with other light infantry. You can read about Caesar doing it over and over again in CDBG.
Is there a way to, perhaps, add "siege tower" properties to cavalry, so that some infantry can "pick up" the cavalry unit, and ride on the horses along with the riders? Typically the movement rate when carrying a siege device is lowered, but I see no reason why it couldn't be raised instead. I'm guessing this is probably the only way to simulate a "mount-dismount" mechanic in the game, and I'm wondering if it would work.
I haven't played as or significantly against the Britons, but they're also supposed to have chariots they can dismount, which daunted the Romans the first time they occupied the island, because they'd use them to outmaneuver the legionaries, but they didn't actually fight while riding on them. They dismounted to engage in melee combat.
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