After some internal discussion with R&D:
1. The main issue in hands is the different number of CPU cores in G8 and G10 models.
2. State sync doesn't work with different configurations and specifically, the different number of cores. That means, it is impossible to have a healthy cluster in your case, regardless of kernel version.
3. We have multi-version cluster support with R80.40. If the number of cores is the same on both servers, you can form a healthy cluster with different kernel versions in R80.40.
4. The whole issue is state sync. As mentioned in another comment, you still can run the cluster, but the state will be Active/Down on one and Ready/Active on the second member. Sync will not work, and in case of a failover, there will be a traffic cut: all connections will have to be reinitialized. If this risk is acceptabe for you, you can go this way, as a temporal emergency measure only, before you can fully recover/replace the faulty model. This by all means is not recommended as a long term solution.
Many thanks to @Gera_Dorfman and @Dorit_Dor for looking into this.