Let’s put it in numbers. You’re both long and short EUR/USD from 1.30. You have a 100 pip take profit limit for each and a 50 pip stop loss limit for each. This is in line with what you’ve described, right?
If the market moves up to your long TP at 1.3100 it will have triggered your short stop at 1.3050 along the way, so you’d have a 50 pip loss on the short and a 100 pip gain on the long - excluding the spread cost incurred by initially putting the trades on in the first place. This is the same as if you just waited and put a long position on at the 1.3050 level, without the initial spread cost.
I’m not sure where the “Sell entry” bit comes in since you were already short to begin with.
Every grid system I’ve seen is a mean reversion strategy. They will do fine in ranging markets but have serious - potentially financially fatal - problems when the market is trending. They can also be more efficiently executed using simple counter trend single trade entries (e.g. long when the market goes up 100 pips) than with these long/short straddle type “hedge” positions.