The only way to do this in QuickBooks is to separate out the item in to two lines. This isn't much of a problem with service or non-inventory parts, but gets harder to do with inventory. You will list the item with the $2,500.00 rate, then add the line for the sales tax rate of 8%. Then add the same item again for the remaining balance and add the item for the second tax rate of 6%.
If you are using inventory, or want your quantities to reflect accurately on non-inventory items, then you will want to make sure you use a quantity of .5 for each item. Some people I have worked with have also broken the percentage out based on the total price. For example, if the total price was $7,000, then the first portion of $2,500 would have a quantity of .35 (2500/7000), while the second item would have a quantity of .65.