I honestly have not heard a doctor say that a membrane gets harder to see the further along you are but maybe its because there was never a question about a membrane with me because my boys got tangled early. That is not to say that your doctor is wrong of course because I'm not the expert I hope he/she is.
Hopefully your doctors will spend some time trying to find that membrane if its in fact there because its presence (or lack thereof) will effect your treatment plan. And of course, there is always the possibility that the membrane did exist but something happened to rupture it. That would make your MoDi twins MoMo twins, complete with all the risks and concerns that entails.
Until your doctor know 100% for sure that there is a membrane in there, I would think you would have to treat your pregnancy as a MoMo pregnancy with the extra monitoring and ultrasounds. MoDi pregnancies have a higher risk of TTTS but a MoMo pregnancy adds cord entanglement and cord compression into the mix.
I hope your doctor is able to give you a more clear answer about the membrane. Even more, I hope your pregnancy continues to go well.