The update brings a new call waiting feature that allows you to accept an incoming WhatsApp call while you are already on another call. However, unlike voice calls, you won't be able to put calls on hold or host a conference call.
Hence, you can either continue with the ongoing WhatsApp call or disconnect it to accept the other incoming call.
The update is now live on Google Play Store and it should arrive for your handset in the coming days if you haven't received it yet. However, if you don't want to wait, you can side-load the stable update (version 2.19.352) from APKMirror.