Try two plastic syringes per nipple. Take the plunger out of one (diameter similar to nipple) and connect it via a short tube to the other, with the plunger in. Place open end over nipple and pull plunger on the "partner" syringe. A dab of KY on the "mouth" of the open syringe helps form a seal (or saliva on the nipple!).

Using non-identical sizes of syringe can create massize nipple suction, ranging from amazingly-stretched to agony.

Locking the plunger "open" is trickier. I put clothes pegs around the exposed plunger to stop it sliding back in.