Can I swap one long shoot day for two short ones and watch the number move?

To cost one long day against two shorter ones while the client is on the phone, build the alternative plan as a scenario. Change the day counts and the overtime in that column alone, and the scenario column header shows what the swap costs against Main. Each scenario is a full independent copy, so the two plans can be genuinely different while sitting side by side.

Cost both plans

With the long-day plan in Main, scroll to the Add Scenario card at the far right and create a scenario based on Main.
Rename the new column by double-clicking its title. "2 short days" beats a number when the client calls back.
In the new column, change the day counts on the affected Cost Items. On a Production Cost Item that means the Prep, Shoot and Wrap days separately.
Adjust the Overtime AddOn on the lines that were running long. Main keeps its long-day brackets while the scenario carries a smaller set, or none.
Read the difference in the scenario column header, beside that column's total, and use the toggle to switch between the money amount and the percentage.

A worked example

One crew position, Fee 800, standard day 8 hours, one overtime bracket to 12 hours at 50 percent. In Main it is one 12-hour day: the hourly rate is 100, so overtime is 4 times 100 times 1.5, or 600, and the line costs 800 plus 600, which is 1,400. In the scenario it is two 8-hour days with no overtime: 2 times 800, which is 1,600. The header reads +200, or +14.29 percent, so the two short days cost more than the one long day for this position.

If the plan is a real option, repeat the steps for a third column and keep all of them.

Edit the right column

Clicking into a field makes that column the active one, and the sidebar follows the active column. The filled dot in a column header shows which column you are editing, so a quick look before typing keeps the change in the plan you mean.

Good to know

Each scenario holds its own copy of the Variables, so a what-if rate in one column never leaks into Main or another scenario.
The sidebar Variables tab can point at any version. Click into that column to make it active when you want to edit its values.
Structure is independent too. Adding a second shoot-day line in the scenario leaves Main untouched.
Cost Item and Section names are set on Main and flow to every column.
For days that differ within one plan, the Overtime AddOn's Individual mode charges only the long days.