Is it posible upgrade version 10.2.1 to 11.6 in two sessions (active-standby system)?

I believe you should be fine, as far as the protocol/service ports used for the Network Failover match (your case) it should work: https://support.f5.com/kb/en-us/solutions/public/9000/000/sol9057.html

After upgrading Device B it will create a traffic-group in standby mode, you should then be able to force Device A to standby and leave the cluster running with Device B 11.x as active, https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/tmos-active-standby-upgrade-11-1-0/1.html

Honestly, I see no point in doing that as the Fallback process is sooo easy, if something goes wrong with 11.x just change the boot location back to 10.x volume.