Please take a look at this question and my answer to have basic understanding on how the manufacturers use the terminologies.
These differences are hardware based and nothing to do with Android. Hence, if you had properly chosen your hardware:
- Yes, you can receive incoming calls in both SIMs without manual switching - even simultaneously! Yes, the other call will be treated as if you are receiving a call when you are on another on a single SIM phone. Depending on your settings and operator restrictions, you may have enabled call waiting and hence it will be displayed on your mobile screen. You can then hold one and switch to another. This was tried in Samsung Galaxy Y Duos phone.
I am not sure about your other question about video call.
Update: In Samsung Galaxy Y Duos (a dual SIM phone) though both SIMs were active to receive calls and access Internet simultaneously, only one of them was allowed to use 3G network. If I opt for 3G network in SIM1 and opt for 3G on SIM2, a message says I can't use 3G on both and changes SIM1's network to 2G and retains SIM2's network to 3G as per my input.