Hey Simon,
Richard gave you a great answer. I will point out some of his words that might be the reason.
In the first Demo account, the EAs are already trading when you activate the second ”fake demo”, which means that there are trades opened in the first account. When the second account gets active, it will open the first trade at the first signal that comes. At this time the trades in the first account are still opened.
Probably the opened trades in the first account ended up profiting, and the first trades in the second account ended up losing.
As Richard said, give it some time, and they will synchronize (when the EA in the two accounts does not have opened trades, the next signal will open the same trade in both accounts).
This is why I check in Monte Carlo to test for the different ”Backtest starting bar”. To make sure that whenever you start with the EA in the account, at one moment, they will synchronize and the overall performance will be similar/same.
Keep us updated on how it goes.