Grow bag advice please

Well that's a first courgette plants producing cucumbers
Cucumbers can be fickle and it may not be vine weevil grubs causing the problem. Too much water can easily cause them to rot and wilt. Too little water will also cause them to wilt and I suspect this may be your problem. Try drip watering from a plastic bottle or similar.
If you do know its vine weevil I would dig it out and dispose of it with ALL the grubs.
I only ever grow 2 cucumbers per deep grow bag so if you remove one you should be fine. You don't mention whether they are in a greenhouse / poly tunnel or outside, mine are in a greenhouse. Once the plants are mature they need at least 2 litres of water per plant each day if it's warm and sunny or they will wilt. The variety I have growing at the moment are Passandra F1 and as a rough estimate they will produce about 25 fruit per plant, maybe more, if they are kept fed and watered. Hope that helps.

Sounds like it has some form of root or stem damage or something preventing it accessing water. Worth trying over watering on the plant in question to see if that changes the situation although in our now winter conditions this could cause an adverse reaction. Slug damage to the stems can also cause the plants to have difficulty taking in water so that's also worth checking. Another possibility is that the adjacent plant has grown healthier / quicker and is absorbing all the water. Again over watering may help prove if this is the problem. Cucumbers like to die at the first opportunity so if the the situation doesn't improve I would have a gentle dig down in the root system and see what's going on underground

I've never had any luck growing courgettes in a grow bag, it's to easy to either over water or they dry out too quickly, always preferred growing either in half whisky barrels or in the ground