Two steps:

1. Get the idx of the position you want to close.
2. Create an order on the opposite direction with the same position size and idx.

This will close the position without touching the other position in hedge.

E.g. If you have a 0.001 BTCUSDT in long position which has an idx of 1, close it by placing a 0.001 BTCUSDT order in short position with idx 1.