With a search you can found that your question is already answered.
But, do you want to send also different trafic from these servers via these public-ip's?
if only this ntp trafic is what you want:
1. Add VIP's as many you have/need
1.1 Go To: Firewall: Virtual IPs and press +
Then add these values:
Type: IP Alias
Interface: WAN
IP Adress: Your additional p-IP with a mask /32
Description: What ever you want
1.2 Apply changes after saving
2. Add Aliases
2.1 Go To: Firewall: Aliases and press +
Then add these values:
Name: Server1 (or second round: Server2)
Description: VLAN # NTP-Server(or whatever you like)
Type: Host(s)
IP: Server internal IP
Description: Server name (or whatever you like)
Now do it again for another server, and after that
NAME: PublicServer1 (or second round: PublicServer2)
Description: what ever you like, but keep it descipive
Type: Host(s)
IP: Server Outside IP
Description: what ever you like
So you end up to have 4 Host-aliases: Server1, Server2, PublicServer1, PublicServer2
2.2 Apply changes after saving
3. Port Forwards
3.1 Go To: Firewall: NAT and press +
Then add these values:
!Notice, that if you find a field which is not told to be touched unnotice those.
Interface: WAN
Protocol: UDP
Destination:
Type: you can either select your WAN/VIP or "Single Host or Alias"
Address: If you chose "Single Host or Alias", then you can type PublicServer1 (or PublicServer2)
Destination port range: from:NTP to:NTP
Redirect target IP:Server1 (or Server2)
Redirect target port: NTP
Description: what ever you like
Now save and do it again for second server
3.2 Apply after saving changes
Before 4th step you need to choose, if you want to have any other trafic also flowing with this additional public-ip
like all trafic from vlan2 to go via p-ip1 and all trafic from vlan3 go via p-ip2
4. Manual Outbound NAT
4.1 Go To: Firewall:NAT:Outbound
4.2 Press Manual Outbound NAT and save
4.3 Press + and after that add these values
NOTICE!: If you find untold field, leave it as is
Interface: WAN
Protocol: UDP (or if any trafic from this vlan, then ANY)
Source:
Type: Network
Address: Either Server1/Server2 (or your vlan subnet) and SM is to server /32 and vlan probably /24
Translation:
Address: Again you can choose WAN/VIP or Host alias(then choose PublicServer(1/2) )
Description: What ever you like
Save
Repeat these to another server
4.4 Change the order of the Mappings, that your server rules is above any other rules
4.5 Save, Apply changes
5. Test that anything works
5.1 Enjoy
In case of something doesn't work Read/search forum before posting. also Documentation might help