The sub will have to be connected to the soundbar, you can't run them separately.
The speakers in that soundbar are so small that they'll give such rubbish sound without the sub.
They way these things are designed, they will only run when connected together, so you can't run one for the TV and one for the hi-fi.
Besides, if your hi-fi is any good, it shouldn't need an additional sub (beyond any it might have already).