Many, if not all, ROM chips have a Chip Enable pin. This also applies to EPROM, EEPROM and Flash. When Chip Enable is low (at or near ground) the chip is activated. When Chip Enable is high (at or near Vdd, 5V in this case) the chip is disabled and all the outputs go to high impedance--they become mostly invisible to the rest of the machine.
So, to do a one switch ROM changer, you would install both sets of ROM in parallel and connect a switch to the CEs. There's a name for the type of switch, but I forget what it is. It has a common and two poles. In one position it connects pole1 to common. In the other position it connects pole2 to common. Use a few hundred ohms of resistance to 5V as the common. Tie each of the CE pins to Ground through something like a 4Kohm resistor or so. That way, they're grounded unless the switch connects them to 5V. The one connected to 5V is disabled and the other one is activated. Voila, one switch changing of the ROMs.
Still, you'll need a way to mount them, but you could just piggy-back them on top of each other, as long as the CE pins are not connected together.
So, to do a one switch ROM changer, you would install both sets of ROM in parallel and connect a switch to the CEs. There's a name for the type of switch, but I forget what it is. It has a common and two poles. In one position it connects pole1 to common. In the other position it connects pole2 to common. Use a few hundred ohms of resistance to 5V as the common. Tie each of the CE pins to Ground through something like a 4Kohm resistor or so. That way, they're grounded unless the switch connects them to 5V. The one connected to 5V is disabled and the other one is activated. Voila, one switch changing of the ROMs.
Still, you'll need a way to mount them, but you could just piggy-back them on top of each other, as long as the CE pins are not connected together.