I would use different priorities. So it will be easier for trouble shooting. In case of troubles you have to check both because you never know which path has been used.

Did you use the test button?

If priority is given 1 both servers smtp will be used? ie incoming mail traffic will routed to both secondary domain

It will be routed to the one which first responds ...

So you will not get one message twice.

if 10 incoming mail are coming how the traffic will be routed.

Say 1 st to node1, 2nd mail to node 2 like vise it will go

So i need a suggestion if i am configuring a LB ie one request goes to one node and 2nd to node 2 will be a good one to work?

here my LB will route the traffic to both nodes

If you use the same priority for both then you are running a kind of LB. However it depends on responsiveness on GWIAs. The faster wins ...

How many mails do you expect per hour?

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).

I understood how it works.

Per hour we will receive around 500.

We are planning to use 2 SMG for inbound/Outbound

That's fine. And one holding the databases?

Both will act as separate SMG