Multi gpu training need advice

hello,

I have 2 gpus (8 + 8)gb. How can i use both the gpu for my training. I am using
model = nn.DataParallel(model)
But, the gpu memeory does not seem to be utilized fully. Is this the right way?

edit: should i buy single 16gb gpu or can i use 2 8gb gpus?

thank you