Is it posible upgrade version 10.2.1 to 11.6 in two sessions (active-standby system)?
The best practice of F5 is upgrade the system active-standby in one session (Upgrading BIG-IP Active-Standby System to Version 11.x).
Initial conditions Unit 1 10.2.1 and Unit 2 10.2.1
But.. In this case we need upgrade one system and keep active with 11.6 (Unit 2) and the other in 10.2.1 (force to standby Unit 1) , the time for this scenario is one week. Finally, after one week, upgrade the system (Unit 1) with 10.2.1 to 11.6. The other question is, Is it posible force to active the Unit 1 after upgrade ?
This kind of upgrade could impact in the performance of BIG-IP and the failover 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.