Hi Arubagirl,
I have thalassemia minor, and during my second pregnancy, my hg also fell to levels similar to yours.  The prolonged anemia during my pregnancy caused my spleen to enlarge and I felt exhausted.  I also had placenta previa, and was at high risk of bleeding - so I was transfused once during my pregnancy.  I was a lot more energetic after the transfusion, and much better able to take care of my son.  My hg held very well after the transfusion, so I did not require another one.  
I think that part of the reason the hg seems lower during pregnancy is that the blood volume increases and the hg appears a little lower, the other may be the pregnancy taxing the body - using up more hg.  
I was lucky to have had the transfusion because my placenta did tear at 36 weeks, and I did bleed quite a bit.  After the c-section my hg was 70.    
As you are carrying twins, and it is earlier in your pregnancy I think it is wise to get the transfusion.  I am glad that I did.    Best of luck!!
Sharmin