Selenium Java code integrating with Jmeter

I have 2 questions:

1. I was able to develop selenium scripts and then export as jar file and then import in Jmeter.

The whole flow worked but that's for only one request/user. if I wanna run concurrently for multiple requests/users it will return me duplicate/failed.

My question is where and how should I create dynamic variable to avoid that problem?

2. is Jmeter right tool to record the UI of single page application for performance testing? it seems like not but please answer with enough details.