Backflow Preventor or CheckValve Needed For Rain Barrel System?

I have a rain barrel system setup that has a transfer pump attached to the rain barrel that routes rain water through a PVC pipe to a spigot in my garden.
I also have another water connection coming from my home's outdoor city water spigot that is connected to the same water line as the rain barrel.

I also have 2 manual PVC shut off ball valves attached to the two water lines so that if I am pulling water from my rain barrel via the transfer pump, I will manually shut off the city water pvc ball valve, and vice versa if I am pulling water from the city water line.

Currently, on the city water spigot attached to my home, I have a vacuum breaker installed, however, after reading more online, I believe another component (i.e. Check Valve / Backflow Preventor) may be needed?

The goal here was that if my rain barrel was low on water, I could manually switch over and use my city water using the same spigot in my garden.

Thanks

Here's a good article explaining the various equipment: Backflow Basics

I think it would be wise to install a backflow preventer.

The main concern is rain water (or worse) backing up into your house (or city) water pipes, and causing trouble. The rain barrel connected to the garden spigot that is also connected to the house spigot, when FULL, the rain barrel could provide back pressure to the house spigot.
According the the article in the link above, a vacuum breaker wouldn't stop that.

The second concern is water siphoning back into the house, from the garden spigot/hose. This, I believe the vacuum breaker does help, and one of the reasons why it is on your house spigot. Also why they are part of irrigation systems, since flooding in your yard could fill the irrigation pipes with unclean water.

You might also go the other route and separate the systems... they can't mix if they aren't connected.

Thanks for the response. I think separating them would be the simplest route, but I was aiming for a more convenient setup so I did not have to unhooked my irrigation system from one spigot to the other anytime my rain barrel was low on rain water.

I’m also looking into ways to automate the system in that condition but needed to figure out if the expensive $300+ backflow preventers are actually necessary for this type of application.

You could fill the tank from the city water with a floating valve, and have no connection between to two water supplies.

Haha, I was literally thinking about the same concept after writing that previous post! I think that is a really smart and inexpensive way to fix my current issue.
Thanks