Jmeter: How can I keep 1000 or “X” users login requests session alive using JMeter?

Here is my Test Scenario:

First, I am generating a load which includes only Login Requests (say for 1000 users). I am using "Ultimate Thread Group" and "Constant Throughput Timer" in my script. Constant Throughput Timer Value=120/sec is used. I want to run this test for five or ten minutes. My load would be held for 3 minutes.

During this Test(while loads are held) , I am sending another set of login requests from the different machine (say for 100 users). I want to measure the response time of this 100 users login which I've generated from the different machine.

But My Requirement is: while I'm sending 100 users login requests, My Previous 1000 users login requests session should be alive on the server. I've also checked "Use KeepAlive" in my login sampler.

So, How can I achieve this?? How can I be sure that my previous all the login requests sessions are held on the server?