Hi, I haven't bf twins but I think you are probably worrying about nothing. If you have got to 5 months without a bottle then you are clearly doing something right!!! It's all downhill from now on. If you don't want to give a bottle, then don't! I am still co sleeping with my 15 month old single baby and he still wakes up once or twice in the night for a feed and tbh, I don't worry about it.
They have another growth spurt around this time. It's not as well known as the others because people used to just wean them. Now research suggests weaning later, the growth spurt is more noticable as they're still just on milk.
There's no way you're not producing enough milk, you've got this far!! Babies do tend to really mess around at this age when thry're feeding, so it's nothing to do with you, it's normal behaviour for a 5 monther and it's REALLY frustrating!!
Hang in there, you're so cloes to 6 mths and you've done brilliantly!