leonardfromoshawa wrote:
. . .
To backup in the future requires hooking up each machine individually and using TM.
To do it that way, yes, but that's not your best bet.
First, having both sets of backups made directly, in the same partition, may present problems later on. See Time Machine - Frequently Asked Question #4 for details.
Second, the inconvenience of switching back and forth means both Macs won't get regular backups. If you do switch frequently, there's the risk of damaging a port or plug.
Your best bet is to partition the drive. Continue backing-up the iMac to one partition directly. Leave the drive connected to the iMac, and back the laptop up to the other partition via your network. See Time Machine - Frequently Asked Question #33 for details, and #22 for setup instructions. That way, both will be backed-up hourly, as long as they're awake and the laptop is in range of your network.
Will my original question of using AirPort Express work
No. You cannot connect a drive to an Airport Express's USB port, only a printer.