I have some criticism to make if you want to improve your design skills, they may sound hard but they are with good intentions.
Seems that you haven't defined the objective of the poster, what you want to achieve with it.
To start you have to answer the following questions
Will it be printed or used on digital media? (that will define the support, is not the same using full rgba colors than just cyan an black)
Which size will it be? (if you use paper you will be attached most-likely to A2, A3, A4, etc sizes)
What is it about? (it's a meeting? About what? It's important to define the objective of the piece)
Which public are you looking for? (non linux users / linux users / 60+ years / teenagers, etc, most of the time this will tell you what kind of arrangements / decorations you can use and what you can't)
Once you define that, you can start doing something.
I let you these links, just to see what other people does, it may give you some ideas, you judge.
You can tell that both systems will participate in the meeting without dividing the poster in two.
You could use the sign/logos of the distributions instead of those two rectangles, and / or put some simple sentences.
Make it cleaner and simple but communicative.
Use a titles, tell an idea, tell something.
And most important give to each element a reading priority.
Start with 2 or 3 levels.
Like a first header & second header and a paragraph+images:
"The importance of the open source
Review of Fedora & SlackwareAnd of course you can add a paragraph with further info.
Or whatever the poster is about. The title is to attract peoplewhich could be interested on the theme of the meeting.
Another important item: don't stretch the logos.
That's my opinion, if I made a mistake I request to my colleagues to point it.
I'm looking forward to see your results
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