Right: for the sake of argument let's assume the price of cable is as follows at the respective time shown:
11:00 - 1.5999 / 1.6001
12:00 - 1.6009 / 1.6011
13:00 - 1.6004 / 1.6006
Now, we go long at 11:00, and Sell at 12:00 - our profit is: -1.6001 (cost of buying cable) + 1.6009 (proceeds from selling cable) = +8 pips
Instead, let's "hedge" our long trade at 12:00...
-1.6001 (cost of buying cable) + 1.6009 (proceeds of shorting cable)...
And, after an hour, decide that the move is finished so we close our original long position as well as covering our hedge
... +1.6004 (proceeds of selling cable from original long) - 1.6006 (cost of covering our short cable hedge) = +6 pips.
So, quelle surpirse, entering the additional trade has cost you the spread (2 pips)