Yes you will need 2 VIO server LPARs and 4
AIX / Linux or OS400 LPARs so 6 in total.
Unless you mean that you want 2 VIO servers and just 2 other OS LPARS in which case you only need 4 LPARS.
Each of the VIO servers and each of the other operating systems will need an LPAR each.
So for two VIO servers you will need two LPARs and then you will need another LPAR for each instance of another operating system you want to run.