Yes, you can. It's equally possible to use IP addresses which are tied to ports instead of addresses from a pool. One possible configuration to achieve this follows.
aaa new-modem
aaa authentication ppp default TACACS+
aaa authorization network TACACS+
!
interface Async1
ip unnumbered Ethernet0
peer default ip address 172.16.1.1
!
interface Async2
ip unnumbered Ethernet0
peer default ip address 172.16.1.2
user = "dynamic" {
....
service = ppp protocol = ip {
addr="172.16.1.1"
addr="172.16.1.2"
}
}
user = "static" {
...
service = ppp protocol = ip {
addr="10.1.1.1"
}
}