It's doable by way of the officers' extra orders, but that just makes the order process slightly less piece-meal — even a Major would only be able to get two vehicle/infantry combined units moved per Turn.
It could be run as a variant of the "mounting vehicles" rules, where the infantry don't actually get into/on to the vehicle and the vehicle is then restricted to the move speed of the accompanying infantry. In that case, if the vehicle is destroyed, it wouldn't also take out the infantry (unless it's H.E.)
Unlike the regular "mounting transports" rules, the infantry in this situation would be able to "mount" a moving vehicle as long as it isn't moving any faster than the infantry. I think I'd limit that to 6" (ADVANCE speed), due to the need to coordinate between the infantry and vehicle to maintain cover.
NOTE: I noticed, while reading about mounting transports, that it explicitly states that a unit can't be split between two or more transports.Anyway, while this might work for the "tank-shield" circumstance, it still doesn't really cater to things like a combined assault. Maybe that's the sort of thing that would best be handled with the officers' extra orders.
This is a STUPID RULE, which I fully intend to ignore. For a start, it would make it impossible to represent the British practice of allotting two Bren Carriers to carry one section of infantry.
Even then, under a strict interpretation of the rules as written, I think the assault would have to be worked through in its entirety when the first unit is ordered in, rather than counting both assaulting units as a single entity for that purpose.