calling a webApi inside another webApi

My initial reaction is "Why?" Since a WebAPI is simply a means to expose an Expression Rule, you could architect your WebAPIs so that the core Expression Rules are re-usable and re-use them as needed.

the core functionality am trying is, i have an Approval application build using Appian,

i need to expose all the request which is pending for approval as webApi, --- which is accomplished

now i want to approve the Request from WebApi, Say we have an Approve and Reject link for each result in WebApi and clicking on them will do the job,

for this i created an 2nd webApi with POST action with RequestId and IsApproved Query Parameters,

how to call this WebApi2 from WebApi1

I think there's some confusion here as to what you're trying to do. Both WebAPIs should be called by a 3rd Party system. That's why you even create a WebAPI in the first place - so that an external system can consume them. It would be the external system's responsibility to deliver the "glue" that joins these WebServices together. 

If you're wanting to deliver a solution that is entirely contained within Appian then you do not need to use WebAPIs! That's an incorrect design.

ya, am trying to expose this to an mobile Application, when an user can check if any approval is ending from his side and approve from mobile

So: design and build your solution entirely in Appian and use the Appian Mobile App to deliver the Mobile experience. 

am sorry Stewart, i guess am not able to put it in proper form, if am not wrong to use Appian Mobile App we have to be in Appian network, what can we do to achieve if we are outside the Organization network

You need connectivity to Appian. There is some limited off-line capability, but you do need to connect in order to  complete any transactions. Is your Appian instance on-premise or Cloud?

cloud

Then as long as you can get a connection that's all you need. Are you Mobile users going to be places where they CAN'T get connections (via WiFi or 4G networks)?