Xcode 10.2 and 10.1

You'd better have only one version (but you can rename the old XCode to keep it ; however, take care of some side effects).

What I do, is keep a beta version of older release, to be able to use it easily.

>I want to install Xcode 10.2

I'll assume you meant v10.2.1...

To answer your 2nd. question first, older Xcode's/.xips are here: More*

As noted, change the name on the older version.

Keep in mind they will both use the same plist preference files, so expect various settings, recents list, etc. to be mirrored. In the case where you have version-specific projects, I'd not open them with both versions...xcode will salt them. Example where an older version Xcode might not be happy about opening/working with a project from a newer version. And while there are ways to recover, it's easier to just go around the minefield, instead.

Be sure to keep tested and reliable backup of your projects should you need to retreat, etc.

*FYI for the community, Apple has once again moved the furniture...the 'More' link used to be at the bottom of the main 'Software Downloads' page, now it's top right. They did keep the actual current version Xcode download via theMac App Store, however.