I have two levies on one fee. Does the second one include the first?

Each AddOn on a line takes the same starting amount, the line's own amount: Amount multiplied by the day count multiplied by Fee. Two levies on one fee therefore both work from the fee, and neither includes the other.

When a levy is due on the fee plus another levy, you choose that on the AddOn that should sit on top, and the budget follows your choice exactly.

Put one levy on top of another

Expand the AddOn that should sit on top.
In its sentence line, click the button that shows what the rate applies to. It reads Base when only the line's own amount is selected, Nothing when nothing is, and Base and 2 AddOns (with the matching number) when other AddOns are ticked.
The menu lists the line's own amount first, shown as [Cost Item name] Base Sum with its figure. Below it is one row for every other AddOn on that line, each with a checkbox.
Tick the AddOn it should sit on. Leave the line's own amount ticked if the levy is due on the fee plus that AddOn, or untick it if the levy is due on the other AddOn's result alone.

Which boxes are ticked when a card arrives is set by the card, and you can change them on any line.

A worked example

One person, one day, Fee 1,000, and two levies on the line.

Levy A at 20% gives 200, calculated on the 1,000.
Levy B at 5% gives 50, also calculated on the 1,000. The two do not compound.
Tick Levy A in Levy B's selection menu, and Levy B now works from 1,000 plus 200. Five percent of 1,200 is 60.

The line total is the fee plus every AddOn result, and the Section subtotal and Total follow automatically.

Good to know

The order of the strips on a line changes how the line looks and never the result. Each AddOn calculates from the boxes you ticked, wherever it sits in the stack.
If two AddOns would tick each other, Splinde spots the loop the moment you tick it and the result shows zero until you untick one, so you always know. Tick in one direction only.
With Unified, the ticked amounts are added up and one rate applies to the sum. With Individual, each ticked amount gets its own row and its own rate, so the fee and another AddOn's result can carry different percentages. The two modes keep separate settings.
When a levy is not among the prepared cards, the blank Markup AddOn takes the same selection menu. See How do I budget my country's employer contributions and levies?.