A compound tax is a tax stacked on top of a primary tax. This is different than just applying 2 different taxes to an item. Here's an example:

Applying 2 Different Taxes:

Item Cost: $100.00
1st tax of 5%: $5.00
2nd tax of 10%: $10.00
Total: $115.00

Applying Compound Taxes:

Item Cost: $100.00
Primary tax of 5%: $5.00
Subtotal: $105.00
Compound tax of 10%: $10.50
Grand Total: $115.50

To apply compound taxes in LessAccounting, you'll need to first figure out your subtotal, then apply a tax (as set in up Settings > Sales Tax) to that amount.