Installing Older Version of MacOS/OSX alongside current
Yes, you can certainly install two versions alongside.

The answer from benwiggy is incorrect in assuming that you cannot have APFS and HFS+ coexisting on the same physical drive. It is indeed possible, and very easily achievable using Disk Utility to create two partitions on the drive. One for the APFS container (which holds your APFS file systems), and one for the HFS+ file system.

Instead of running 10.11, I would recommended having 10.14 and 10.15 coexisting. The reason being that support for 10.11 ended a year ago - this means that if you want to connect it to a network, you'll probably end up with security related issues.