Is it possible to use true polymorph to transform 2 PC characters, each of whom is at least level 15, into a CR 30 Tarrasque?
No, this will not work
True Polymorph has no inherent ability to combine creatures as it is written and cannot be presumed to do so.
The closest I could get to seeing this being possible is having a Sorcerer cast True Polymorph from a scroll or other source and use the twinned spell metamagic. This effect applied to your situation would polymorph your two level 15 PCs into two of the same monster but they would still be separate monsters of CR 15 or lower. Twinned spell applies the effects of the spell to two targets but does not change the spell's text.
A similar question addressed an issue like this here: Can I make independent spell-effect choices for a Twinned spell?