In the episode, it's stated that Flash and the other speedster must work together, adding Barry's speed to the other's, in order to open a time portal.
As you noted, Barry's been able to open time portals on his own before; however, as the earlier respondent stated, this was never something that he'd done on purpose (or, at least not until another few episodes, when he goes back in time to get Season 1 Harrison Wells to help him increase his speed).
So, it's not terribly clear why both their efforts were required to send the other speedster back through time.
A couple of possibilities:
- They're basing their theory on what the other speedster has stated; that information may not be correct (whether by mistake, or by design).
- The other speedster needed the additional speed to get the time portal open for some reason: he's just a little slower than Flash, or the distance he must traverse, or due to the presence of the Time Wraiths we learn of later.
NOTE: there's another questionable element here. It's not at all clear how the two speedsters, running at roughly the same rate, can have one push the other and thereby double his rate of speed. In the comics, speedsters have sometimes managed to use the Speed Force to temporarily give others super-speed. Perhaps that's what actually happened.