It's a bit different in FX trading, after you open a position there are four variables that are important to you: , equity, used margin (constant), free margin and margin level.
Margin (used) is position size divided by leverage,
Equity is Balance - Floating P/L (profit/loss - swaps - spreads - commission)
Free margin is equity - used margin.
and finally, margin level = used margin/equity
When you add a new position, respective used margin and floating P/L is added to the used margin and floating P/L of the first position. Equity and margin level (for combined position) is recalculated according to new values of used margin and aggregated floating P/L.
As far as I know there is no such thing as maintenance margin in CFDs trading. You are required to keep margin level for all your positions combined above 50%. Dropping below 50% you get margin call, if you reach 30% you get stopped out.