...Any idea on how this could be achieved without using two modules (to reduce power consumption)?
Assuming that you wish to automate what would manually be a 2 button system, basically no. Power consumption shouldn't be an issue, however, as only one module is active at a time and then only for a few seconds. The real issue,
after working out the interface, is where to put it. For convenience, a pair of macros could be written to say OPEN with an ON and CLOSE with an OFF but you would still need 2 physical modules. Two modules are a major wall wart.
...Could you be a little more specific about how the curtains operate now...
While I believe the operator control functions for most automated curtains is similar, there could be some major circuit differences between manufactures.
I'd partially want to know if there any kind of auto stop when they reach an OPEN or CLOSED position. i.e. If you're manually pushing a button, it's easy to see when to stop. On the other hand, if have X10 signaling an ON for 30 seconds, it might be an issue if the curtain is trying to move after it's reaches the "End of the Line". Here again it is important to know how the curtain works.
Drifting off to dream land...If I were designing basic, non automated control, specifications for a curtain, I'd recommend a SPDT momentary rocker switch with a center OFF position. I'd have a one second push of a button do a full OPEN or CLOSE respective to rocker switch position pushed. Holding a button for more than 1 second in my ideal world would move curtain till released. It would have an "End of Line" shutoff for both cases. I'd then have the lawyers word something in the disclaimer section cautioning against modifying any hardware. After that, I'd have my engineering department design an automation plug in.
Anyone have a lot of venture capital they need to spend? I expect I'll need to have a few research retreats during the design and testing phases of production for "beta site testing".
Waking up/Back to the real world...There have been some very detailed discussions on this subject in the past. You might benefit from reading how others have already approached the issue.
One major caution: Make should your design does not allow any possibility that an OPEN and a CLOSE signal can be sent at the same time.
Drifting off to dream land, again...What I'd like to see is a retro fit attachment to control blinds. In my dream world, it could sense and adjust the blinds for the position of optimum light entry.