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In my fictional universe there are two galactic civilizations that have never interacted until recently and thus have developed different cultures, science, etc.

from a technological standpoint, one of the civilizations is similar to Halo's UNSC and the other is straight up star wars

What prevented them form ever interacting is a "natural" barrier of dead worlds making the trips longer and more risky with the lack of any ports/refuelling stations.

There was never a unified galactic government that just split and forgot the other.

And these dead worlds have no possibility to hold life at all

So my question is: would the dead world divide would realistically stop the two civilizations form ever meeting just enough so that two galactic civilizations develop differently

You need more than dead. You need dead and weird.

Interstellar spaces are huge. Ships need to be able to traverse these distances. The fact that there are clumps of matter - "dead worlds" - in between that are not useful to these ships should make no difference. It is like the fact that there are some empty office buildings on my route home. Who cares? You can't pull over for lunch at a star or a black hole either. Also, even if they did need to pull over from time to time, I cannot imagine why they would need a life filled world. These ships don't burn wood. If they need dilithium crystals or something they go where those things are.

You need a barrier that is not just lifeless and useless, because presumably lifeless and useless is most of everywhere. The barrier needs to pose some existential threat to the ships that want to get near it - something active that poses a hazard. And it needs to be a unique hazard - these spacefarers will know about radiation, know about asteroid fields, know about gravity wells. It needs to be something that does not occur in their territories and that is hard to study and understand.

I propose that you have spacetime itself be unpredictable within the dead zone. The fabric of space is full of holes and the laws of physics do not reliably apply. Weird stuff happens. This is also why these civilizations are not aware of one another - electromagnetic radiation does not reliably traverse the dead zone, and what does get across is changed and different once it does.

When they finally do get in touch it is because someone took the long way around and found out what was on the other side. The long way around is still a barrier but at least you know what and when you are when you finally finish it.

Yes, but it will depend heavily on the values of each civilization.

A pragmatic society will only visit solar systems they expect to be worth the return on investment based on what they already know. They will do years of analysis for hazardous solar activity, scan for signs of existing civilizations, search for promising exoworlds, and send unmanned probes, all before they ever risk sending a ship on a mission to any star system. They will be very selective as to where they go just to be good and sure it is safe and profitable; so, they will rule out the dead zone quickly and go the other way.

However, a more idealistic civilization will care more about exploration for exploration sake. They will be intrigued by these dead worlds. They will want to chart each one just to see if there are new kinds of life there that they could have never predicted with their current understandings of science. They will want to know, "what is on the other side". Such civilizations are much more willing to risk a multi-billion dollar ship and the lives of all it's crew exploring worlds that are not expected to have a measurable return on investment.

Yes this is a realistic possibility.

If Earth ever develops a probe that can travel long distances. You can bet we will pick a target where we think life exists, probably the direction that involves passing the most candidates as possible.

If we find life, that will probably be where we focus all our efforts.

So for your galactic civilizations, you just need something more interesting in a different direction as to distract both civilizations from ever meeting each other.

Probably not.

Galactic civilizations require a lot of time to develop. If you have a growing civilization that grows in the span of 100.000 years then the light of their activity has also reached 100.000 lightyears of space. A quick google search of the first 3 Galaxies show them anywhere between 30 to 100 lightyears in size. So at the very least these civilizations know of each other and have attempted long-range communication at the very least.

And expanding across the stars even with FTL takes a lot of time. You are better off saying these are in seperate Galaxies and the space between them has gotten smaller or the technology good enough to travel between them.