The big problem is on the other machine. Let's say you get (public) IP addresses 10.20.30.40 and 10.20.30.41 and you use 10.20.30.40 for router's own WAN address (bound to pppoe-out1 interface). If you configure another box with 10.20.30.41/32 ... you need to tell it which IP address has its upstream router (that's your mikrotik) and configure ethernet in a point-to-point manner. It can be done, but I'm not sure what's to benefit by having end device set up with public address.
I'd use the extra address as another possible IP address in NAT process while keeping server on private address. If server uses address 192.168.13.13, then NAT rules would be something like this:
/ip firewall nat
add chain=srcnat action=netmap src-address=192.168.13.13 to-addresses=10.20.30.41 out-interface-list=WAN
add chain=srcnat action=masquerade out-interface-list=WAN ipsec-policy=out,none
add chain=dstnat action=netmap dst-address=10.20.30.41to-addresses=192.168.13.13 in-interface-list=WAN
Could be, that rule order is not important, but it doesn't hurt to have srcnat netmap above srcnat masquerade in case that rule search strategy is same as in the rest of firewall (top to botton, first one matching executes).
In this case you probably don't have to anything else with regard to the extra IP address (pppoe means that packets are routed towards to your router and it's not necessary to play games with proxy-arp or some such).