Hello
If can cop designing with some logic, then using a small reed switch will enable you to open the curtains to the half, another one to open curtains completely.
The transmitter can be some kind of RF, or IR. When you first press the open button on your Tx, the motor will work, until it press the first reed switch, which in turn rests a flip-flop for example, pressing the Tx again, will make the motor play, and open the curtains completely.
I think using usual motors is much easier than controllinh stepper ones.
Regards