What hardware do we need to share one printer between two independant networks, each is associated with a separate busiess
When you say two independent networks, do you mean in separate buildings or two different networks in the same locality? What equipment are you using? There's no magic "printer bridge for different networks" you can plug in to have it work.
If two different buildings you could try using a VPN to link the two networks (what are you using for routing/firewall?) Then anything being sent to that printer IP (is it a network printer?) would get routed to that network and printed.
If in the same physical area you would have to go where you're separating the networks and set up the router to see the two networks (or the switch controlling the VLANs) and configure it to allow a static route to the printer.
In either case the printer would need a static IP.
Or you can rely on printing the document to PDF and emailing it to someone in the network with the printer and having them print it, or you can make lots of people smile and print the document to an iPad's iBooks application and try sharing them as PDF eBooks. Those could work too, but my boss won't listen when I propose that.
You could try using IPP.
You don't state any specifics about the networks, printers, or client OS'es involved so I'll just leave you this Wikipadia link for some basic info: