Alright I’m stumped.
I have two racks in a DC that currently is connected through a direct link. Each of the Mikrotik Routers have their own WANs with /24 IPs. What I need help with is I’ve got both Mikrotiks connected and communicating. I need to some VMs that live in Rack 2 to use WAN IPs on Rack 1 until I can decom that whole Rack.
Let’s start from an explanation what a “direct link” means in case of your datacenter. Is it a cable, or at least an L2 interconnect provided by the DC? If so, you can bridge the LANs of the two Mikrotik routers together using that “direct link”, and use the one in the Rack 2 as a mere switch. If it is an L3 connection, you can still use an L2 tunnel to do the same. L2 tunnels are not a preferred choice in general but if it is a temporary solution, it is the simplest one here.
When you bridge the two LANs together, you can still use both WANs if the LAN IP addresses of the two Mikrotiks are different, so you can change the the networking setup of the VMs one by one. Even if the LAN IP addresses of both Mikrotiks are the same, you can still use both if you manually create the ARP records on the VMs so that you can control which VM uses which Mikrotik, you can even do that without restarting the VM (but each change will kill the ongoing connections of the VM). But putting up a different address up on the Tik in Rack2 first in parallel to the existing one, changing the routes on the VMs (which can also be done on the fly but without affecting the ongoing connections as you don’t change the WAN in this case), and only then removing the original address and bridging the LANs together is a cleaner way.
The details of the migration plan depend on what the current setup is, how many short-term outages of the VMs you can afford, and whether your virtualization platform allows live migration of the VMs between physical hosts.