Assuming a very simple network;
Internet===<PPPoE>===[MikroTik]===LAN (including host you want 2nd public IP routed to)
assuming that you will NOT be doing filtering or NAT/Masquerading on any packets with the 2nd public IP as either SOURCE or DESTINATION address;
assuming your ISP has correctly routed the 2nd public IP to your 1st public PPPoE IP;
AND assuming the internal (LAN) host to which you want the 2nd IP routed has BOTH a local LAN IP address that won't change AND also is configured with the 2nd public IP;
THEN the solution is as simple as adding a single static route:
/ip route add dst-address=2ND.PUB.LIC.IP/32 gateway=LAN.IP.OF.HOST
If on the other hand, the internal LAN host ONLY has a private LAN IP, then you will need to set up NAT rules.