Basic routing for subnets between two routers

What routers do you have?

Some ways to go about this like site to site VPN but for a non-VPN setup I show you that way

Not sure why you x out the IP unless they are real WAN IP subnets and not like 192.168.15.0/24 ?

Depending on the model USG you have you want the connect two routers in a external or general interface type link but this this might be done under internal but I do it this way.

So USG 1
LAN port with 192.168.15.0/24
on another port not shared with your LAN port check your port role setup this depends on your USG model
so on LAN2 make a VLAN say 99 external with a subnet 192.168.99.1 255.255.255.252 gateway 192.168.99.2
you can make a zone like uplink for this VLAN and interface name uplink

on USG 2
LAN port with 192.168.200.0/24
on another port not shared with your LAN port check your port role setup this depends on your USG model
so on LAN2 (base port depending on model) make a VLAN say 99 external with a subnet 192.168.99.2 255.255.255.252 gateway 192.168.99.1
you can make a zone like uplink for this VLAN and interface name uplink

On USG1 go to routing static routes with the following
IP 192.168.200.0
subnet 255.255.255.0 I'm guessing this is the right subnet?
Gateway 192.168.99.2

On USG2 go to routing static routes with the following
IP 192.168.15.0
subnet 255.255.255.0 I'm guessing this is the right subnet?
Gateway 192.168.99.1

firewall rule on both ends
from LAN
to uplink

and
from uplink
to LAN

depending on traffic flow

I don't think you need to do routing rules but here they are

On USG1
incoming LAN for 192.168.15.0/24
destination 192.168.200.0/24
next hop gateway 192.168.99.2
Source network address translation none

On USG2
incoming LAN for 192.168.200.0/24
destination 192.168.15.0/24
next hop gateway 192.168.99.1
Source network address translation none

You say the two USG are physically next to each other so be such to link then together without going by a switch as the uplink is by VLAN unless you setup a switch to tag VLAN99 but I'm guessing you have no need to do this.