65% attenuation seems low. If it were me I would add half the T58 before bottling to ensure it's not going to eat more sugars than you expect and you end up with gushers, or worse; bombs.
I'd probably add half the T58 packet to a couple of liters of average wort (about 1.060) and let it start fermenting (a couple of days), then add a couple of liters of the IIPA to it and let it ferment for a day or two, and then add all of it to the main batch and see what happens.
If it ferments lower than 1.035, then you saved yourself some real problems. If it stays where it is, you know it is ready to bottle.
I did this to a 1.100 barleywine, and it took it from 1.025 to 1.017. Certainly saved me from a big mess with exploding bottles.