When you open both a long and a short position in a currency pair, you hold locked positions.
The margin requirement for the locked positions represents that for the position with a larger size.
For example, if you hold a long position in 1 lot of EURUSD with a margin requirement of 5,000 USD, and a short position in 0.5 lot of EURUSD with a margin requirement of 2,500 USD, the margin requirement for your locked positions will be 5,000 USD.