。
Let's have a frank discussion.
Literal
Belly [object-を] split-and talk-[volitional-しよう].
話し合おう ('let's discuss') is the volitional of 話し合う — the 合う adds mutuality, framing it as a two-way exchange rather than one-sided talk. The idiom 腹を割って ('split one's belly') means speaking with complete frankness, holding nothing back.