Maybe it interesting who initiated talk after communication breakdown? Frigid Onanoko guessing it be girl. People, be less harsh on girl! Just because she is girl, that no mean she know how she want to position herself between different people. She need time too, to understand own feelings. It not so nice for Op, but can you expect better from her - is she really at fault? (this in reaction to >>7 btw)
What FO hear is, girl is also positioning. She like both Op and other guy. If wholly choose for one guy, big chance to lose touch with other person. She wanna know for sure she no cut loose one guy (the other one) only to find out that relation with nr 2 didn't work out after all. This is the game, and FO think girl afraid of losing twice.
She will need to make choice sooner or later, because it also eating on her. She is free woman, that choice is hers. Not yours. Not of other guy. Now for advice. Gonna assume here that you two not meet yet.
Make sure you meet her - it no need to be big (remember she is de jura 'taken') - but insist on meeting. It send signal that you at least giving it a chance (which you are). It also necessary to test waters - how do you and she behave when together? This be very informative to both of you and perhaps conclusive decision formed right there (or at least roots of.)
Depending on how conversation goes (topics of your choice) make clear that communication of past weeks rekindled flame that burn before (Frigid Onanoko so romantic, nya!) from cinders of the love that fell unattended during these five months. But she also started some fire elsewhere, or so you heard. Girl, she must extinguish one of them. You cannot guarantee the remaining fire will burn forever, but as far as you concerned, that irrelevant. Make sure you press that its her choice. No need to press her, the answer will come eventually when she knows you expect one.
Tell her one more thing. If she does not extinguish your fire, but keeps feeding the other flame, it will burn the bridges between you beyond repair.
Frigid Onanoko can't guarantee anything, of course. Assuming 25% chance of success, maybe less. Meet her to at least know if you really want her. saying bye now!