How best to keep these occasionally polarizing aims balanced without
creating reader whiplash?
This is character conflict and it's a good thing
I think it's not about which character is "winning" at any given moment, but about this trust dynamic between the characters.
Over the course of the plot, each character will need to negotiate their own level of habitual nature, short-term goals, and long-term goals. There's something they want/need in both the short term and the long which makes them willing to form short and long-term alliances, but there's also something that triggers them – a personal breaking point.
Story-wise, the breaking point won't be an accumulation of circumstances, it will be a specific thing that "breaks the suspension of animosity". It's a specific story beat, which is probably built up to or repeated as a pattern. The character can put up with a lot, but this one thing is a line that when crossed, the alliance is over. The characters won't telegraph it to each other – they will keep their game faces – but the reader should pick up when that line has been crossed. The sub-text changes, or the character begins to act with suspicion or caution.
This dynamic has it's own arc that you can roughly pace against the timeline of events. It might look something like:
- Enemy agents, anonymous distrust
- Personal encounter, direct animosity (They shoot at each other.)
- calculated negotiation (surrender and you will live.) long-term/short-term compromise
- long-term goal, and anonymous trust (As a members of law enforcement, we have reasons to work together.)
- long-term goal, personal trust (Working together, seeing the other is competent)
- long-term goal, but a trigger leads to personal mistrust (trust erodes)
- short term compromise (work together for now, but look for exit strategy)
- Other character's trigger (the discovery of an exit strategy undermines trust in both long and short term goals)
You are moving the trust "needle" through the various stages. Build trust with shared long and short term goals, break trust with triggers but negotiate with long vs short term goals.
The characters won't act until an opportunity arrives, and they won't telegraph their trust level to the other characters ahead of being able to do something about it. Use this shifting trust dynamic to justify the twists in the plot, not the other way around. Signal the trust level ahead of plot twists and sudden character turns. The reader won't get whiplash, the story will start to make sense because the character's behavior is consistent with their goals and triggers.