what happen if we try to create HA between 2 different BIGIP model?

Hi,

For information I already do it and it's working.

Beginning in BIG-IP 11.4.0, F5 supports redundant devices in a device group on different hardware or virtual platforms through the load-aware failover feature. For more information about load-aware failover, refer to the Managing Failover chapter of the BIG-IP Device Service Clustering: Administrationmanual.

for more information:

https://support.f5.com/csp/article/K8665

regards