Im trying to work out the following

One company has a Draytek 2765 Router connected to the internet with a block of 5 Public IP addresses and another company want to share the internet connection using another Draytek (2862) router making the 2 companies totally separate. With a spare IP is is possible to connect to one of the ports on the 2765 router on give the wan side of the 2862 one of the spare public IP addresses so that no double natting occurs

Easiest way is not to use a separate router but to use one of the public ips and one of the ports with a separate subnet and no inter lan routing. port forwards etc are under the control of the router admin but all traffic segmented and no double nat.