Hello Shan,
This is the normal behaviour of the system. Since there is difference in these two tax condition types, the posting also will be as seperate line item in accounting. Please see the pricing at header level of billing document, you see the condition types will be posted to accounting. At header you can find the cumulation of unique condition types at item level pricing. Since these 2 tax condition types are different, you can find two condition types at Header level pricing tab. Hence posting as seperate line items in accounting.
Warm Regards
Siva B