How to dual boot Mojave / Catalina

Apple beta products are outside of the support scope in the Apple Support Communities. Apple does not recommend that you install the beta on a production Mac. Anyone that may have already installed the Catalina beta cannot discuss it here, as that would be a violation of the beta agreement, and the terms of this site.

Well, my understanding is that the developer beta may not be exaxtly the same as the public beta and the developer forums are exactly what their name implies: they are for developers, not a general discussion of public beta problems. My info comes from these

https://beta.apple.com/sp/betaprogram/faq

and the included link for the terms.

And here are the terms for the developer forums which are also considered confidential.

https://forums.developer.apple.com/create-account.jspa

There is a third program which is by invitation only and that does have its own discussion forums which are also governed by an NDA and cannot be discussed here.

I don't do dual boot solutions here, so someone else will have to address your question, especially if it pertains to APFS.

The quanta of individuals that use n-tuple boot operating systems on native Mac hardware is a very small number, and even smaller for those that reqularly respond to user questions here. If you get in a jam configuring your Mac in an Apple unsupported manner, you may endure a very long wait for help.

I have an iMac with an internal SSD, and a dedicated external 512 GB Crucial SSD in a USB3 enclosure that is formatted HFS+. On that external drive, I keep my Parallel's OS X El Capitan guest, and two VirtualBox guests (Win10, and Ubuntu 18.04). It is not dual-boot, but the performance over USB3 is sufficient with all SSD activity that it is quite usable.

Basically, what you need to do is create a separate APFS partition,

not a container. Click the icon in the upper left corner of the Disk Utility

box somewhat it is set to show devices. Then select the drive, not any

volume under it. Then click partition and in the display click the "+" to

add a partition and size to appropriately, chose APFS for the file system.

99% of the time there will be no issue, however, it is always a good idea

to do a full system backup just in case

I did this when the public beta for Mojave came out and I was running

High Sierra as my "main" system. FWIW, I started on an external drive but

I had issues with applying updates to Mojave Public Beta on the external.

When I moved it internal, the update issues went away. Perhaps Catalina

will resolve those sorts of issues.

You might also want to remember that Apple considers betas to be confidential, so you will not be able to discuss any problems here or anywhere else and we are not allowed to help. In other words, you’ll be on your own with your only remedy being to report problems to Apple via a built in Feedback Assistant.

FWIW, one can discuss beta issues, even public betas, in the specific

developers only communities but not outside in these public communities.

So, one is not totally on their own. Though, when I encountered issues, I

personally never found much help there. You also don't have to be a "paying"

developer to do so. Unless these policies have recently changed.

Perhaps I may have been in group 3 as all the discussions were all

related to the public beta and issues with the various parts of macOS

(i.e. Mail, Safari, Photos, updating, App Store, etc.) and none were related

at all to developer specific issues.

Or maybe my developer account which was opened before OSX was

even imagined granted me access.

Regardless, yes, nothing discussed in those communities was permitted outside

of those communities else lightning would strike you down where you stood

or even worse.

You would know if you were in the third program - it is by invitation only directly and personally from the Apple program and there are no discussions at all re. the public beta or the developer betas. You need to be specifically invited, accept the invitation, register the device serial number and specs with which you will be testing, sign an NDA, and resign from the project if or when so desired.

Just to add, the Public Beta will not be available until sometime in July.