None of the answers are quite right. Two people (not one) were presumably sat next to Silent at Norwich.
For each person, assuming that everyone at Norwich also went to Barnsley, the odds are 2 / 4,999 that they would be sat in one of the seats either side of Silent Blade. So, the chance of at least one person being sat next to Silent at both games are 4 / 4,999 = 0.0008 or 0.08%. If you prefer odd, 1,995 to 4.
In reality, it's more complex because we don't know how many people attended BOTH matches (if they were completely different sets of people apart from Silent, the chance would be zero, of course). It would also be made a bit more complex by seating arrangements, how they sell blocks of tickets, etc. For instance, if they tend to put 'single ticket' people together, the odds would increase / decrease accordingly.
Let's not open the
can of worms again though.