My understanding of multi Sim  is as follows:

With multi sim you get  two sim cards with the same telephone number.  You then have to designate the two cards as Card A and Card B.
Card A will be your primary card  and can make calls and receive calls.  Card B will be your secondary card but can only make calls but cannot receive calls.  Most users use  card A on their phone  and  card B on their  say IPad since you only need to connect to the internet  and not receive calls.

The allocation of data or voice plans then has to be according to your needs and budget.  Since i was an early bird on multisim  ,  both my sim cards have not only voice but unlimited data on both Sim cards which i'm told is not possible currently as unlimited data plans are not marketed nowadays.

Cost of Multisim with voice and unlimited Data  is  RM15  ( for multisim )  and RM99 for unlimited data on both cards . Total RM114 per month.

By the way i'm using Maxis and cannot speak for Celcom or DIGI. Size of Sim card  is dependent on your gadget and nothing to do with any plan.

Hope to have been of help.

Cheers

Herojam

The answer to your questions:

Your Question 1:  yes you can  but you will not be able to receive any calls on Card A  if this card is on your phone as both your devices will be connected to the internet  at the same time.

Your Question 2 :  When you first get the Sim cards , you have to designate which card is Card A and which is Card B.
That is :  Card A  can make calls  and receive calls .  Card  B cannot receive calls but can make calls.  Both can connect to the internet at the same time if you want  but thats  defeats the purpose  as you will not be able to receive calls on Card A.

Hope i have answered your questions.
Cheers

Herojam