That's what Informatica has invented the High Availability (HA) option for. Which you perform maintenance actions on one server, the other server(s) is/are up and running and execute PowerCenter sessions and workflows.
The only thing you can NOT do is to run different Informatica versions within one single Informatica domain; the Informatica domain requires that all nodes in a domain run the same Informatica software version.
In order to make this work you will at least one other license option for PowerCenter, namely Server Grid; together these two options (HA and Server Grid) give you what you need here.
You will need at least two nodes both acting as gateway nodes.
You will need to set up Integration Services running on the server grid instead of on an individual node.
You will have to set up the HA prerequisites, in particular a shared file system which is supported with HA.
And it is necesssary that mappings and sessions and workflows can be executed on the server grid as well. For example, if you have a Stored Procedure (SP) which must be invoked in some particular order during the mapping, then chances are that this process cannot run distributed over a grid but only on one single node.
This is a pretty wide topic, but the outlines above should be enough for the moment being so that you know where to start.
Thank you for the response. Have option of HA and there are two nodes(primary and backup). Do we have any documents which we can refer to implement this.
There should be some Best Practices articles available in Velocity (the Best Practices collection maintained by Informatica Professional Services, to be found under the link Resources at the top of the Informatica Network home page) for HA.
Also this has been discussed a couple of times during the past years. You should be able to find quite a few discussions in the PowerCenter forum.