can i open a buy & sell trade on same instrument simultaneously?

Yes, you can open both buy & sell positions at the same time and for the same instrument. This practice is known as hedging.

Let me add that when you are opening your trading account you may get the option to open a hedging or netting account.

On a hedging account, every trade is displayed as a unique trade with a unique transaction ID. So, if you open 3 buy trades and 3 sell trades on same instrument, the 3 buy trades will be displayed separately and the 3 sell trades will also be displayed separately.

On a netting account the 3 buy trades will be summed up and displayed as one buy trade. while the 3 sell trades will be summed up and be displayed as one sell trade.

Right now it may not sound like a big deal but when you are analyzing your account statement at the end of the month, it makes a whole lot of difference.

You can open buy/sell at the same time and this strategyof trading is used to manage risk. If the buy is making a loss, the sell will be making a profit so when you net off the loss will be reduced.

Yes, but it is mainly a risk management procedure meant to reduce the losses you are already sustaining on a current trade. It is a strategy used for lowering losses and not for making profit.