There are 7 different Classes in Phoenix Point and each of them have unique Skills and attributes. The current available classes are:

Basic Classes - Heavy, Sniper, Assault.
New Jericho (faction) - Technician.
Disciples of Anu (faction) - Berserker, Priest.
Synedrion (faction) - Infiltrator.

All this Classes may combine in 21 dual class combination:
No matter what class get first. Example - Heavy/Sniper=Sniper/Heavy.

01. Heavy/Sniper
02. Heavy/Assault
03. Sniper/Assault

04. Heavy/Technician
05. Sniper/Technician
06. Assault/Technician

07. Heavy/Berserker
08. Sniper/Berserker
09. Assault/Berserker

10. Heavy/Priest
11. Sniper/Priest
12. Assault/Priest

13. Heavy/Infiltrator
14. Sniper/Infiltrator
15. Assault/Infiltrator

16. Technician/Berserker
17. Technician/Priest
18. Technician/Infiltrator

19. Infiltrator/Berserker
20. Infiltrator/Priest

21. Berserker/Priest

All this dual class combination you may get in one game.