You can do it if you like, but just be careful.
Let's say you've got 1 minilot open on a pair. Things have gone very well and you've got 50 pips profit locked in with a stoploss. Opening a 2nd minilot with a 50 pip stoploss would mean that you are risking 100% of your profit in order to hopefully make more money. That's probably not a good idea whether you are trading long term or not.
If you want to add, risk a reasonable percentage of your locked in profits, not your current profits. In the example above, if you really needed a 50 pip stoploss, then the 2nd trade should be for some fraction of a minilot. It's up to you to determine what percentage of your locked in profits you want to risk. Personally, I never risk more than 50%, so in that scenario, My second position would be 5 microlots or less. Another thing to consider if you've opened a 2nd, smaller position would be to move the SL to breakeven more aggressively. This increases the chances of getting stopped out on the second part of the trade, but also does a better job defending existing profits.
If you have a broker that likes to slip you on your stoplosses, take this into account when determining how much profit you really have locked in. (also, consider changing brokers). Using this method, even if the market reverses and slams straight into the stoploss on both trades, you've still got something to show for your time and effort.