However if you go for load balancing and reliability then you have to think of more steps in your environment: i.e.  two mx records, more than one SMG (a bundle of SMGs working together).

We are planning to use 2 SMG for inbound/Outbound

That's fine. And one holding the databases?

Both will act as separate SMG

Both will act as separate SMG

Why separate SMGs?

So if you have larger environment my idea is to do it in this way.

smg1 is holding all databases (configuration, quarantine,tracker, statistics)

smgin1 (maybe add smgin2) for inbound

smgout1 (maybe add smgout2) for outbound.

If your intention is to check mailboxes directly via imap then add another smg for imap activities

In this environment all services report to one server. It is easy to move services from one smg to another one. It is easy to check tracker information or read quarantine, because there is only one database.