I cannot reproduce this issue and am able to execute the nn.DataParallel script multiple times:
# tmp.py
import torch
import torch.nn as nn

model = nn.Sequential(
    nn.Conv2d(3, 6, 3, 1, 1),
    nn.ReLU(),
    nn.Conv2d(6, 12, 3, 1, 1),
    nn.ReLU())

model = nn.DataParallel(model).cuda()
x = torch.randn(8, 3, 224, 224).cuda()

for _ in range(100):
    out = model(x)
print('done')

# run.sh
python tmp.py

# in terminal
bash run.sh &
bash run.sh &
bash run.sh &
bash run.sh &
bash run.sh &

Using nvidia-smi I can see that each device will be used by 5 scripts.