limit speed of 2 IPs using iptables

Seems like you need some traffic shaper like tc.

Maybe you'll find that article somewhat useful

You can use OpenVPN shaper:

       --shaper n
              Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port.  If you want  to
              limit the bandwidth in both directions, use this option on both peers.

              OpenVPN  uses  the following algorithm to implement traffic shaping: Given a shaper rate of n bytes
              per second, after a datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of (b /
              n) seconds before queuing the next write.

              It  should be noted that OpenVPN supports multiple tunnels between the same two peers, allowing you
              to construct full-speed and reduced bandwidth tunnels at the same time, routing  low-priority  data
              such as off-site backups over the reduced bandwidth tunnel, and other data over the full-speed tun‐
              nel.

              Also note that for low bandwidth tunnels (under 1000 bytes per second),  you  should  probably  use
              lower MTU values as well (see above), otherwise the packet latency will grow so large as to trigger
              timeouts in the TLS layer and TCP connections running over the tunnel.

              OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.