We purchase live in our primary house July 2016 - March 2018. in Aug 2018 we purchased another primary house. is the house A still consider as primary house?

Which one is homesteaded ?   That is usually the deciding factor.  

If you sold your primary personal residence and you lived in and owned the home for at least two years in the five year period on the date of sale, you do not have to report the sale if your gains are less then the exclusion amounts of $250,000 if filing Single or $500,000 if filing Married Filing Jointly (and both lived in the home for two years).  

It probably doesn't matter.  If you sold the house and moved because of a job change, you are still allowed an exclusion of the capital gain, even if you did not meet the 2 year rule.  The Maximum exclusion ($500,000 married) is reduced for the time shortage.  So,  if you lived in the house 20 months, your maximum exclusion is 20/24 x $500,000 = $416,667.

That said, your old home is still you primary home until you establish a new primary home (or the old home is sold, whichever comes first).  If you had only temporary living arrangements (e.g. hotels)  April -July 2018, at the new job location,  then your old home was still your primary home, during that period.

the first house must have been used and owned for at least two years (730 days) out of the 5 years before sale as your principal residence. both of you must meet the occupancy test (for the maximum exclusion) but only one needs to meet the ownership test,  the issue is do the days from March to August count. the IRS does not give much info but here is info from IRS PUB 523 home sales.

If you were ever away from home, you need to determine whether that time counts towards your residence requirement. A vacation or other short absence counts as time you lived at home (even if you rented out your home while you were gone).

There is nothing tricky about the language “primary residence”. Your primary residence is where you live most of the time.  If you are audited, because you have two homes and live in both of them, the IRS is going to look for factors like, where do you get your bills delivered? Where do the schools send your children’s report cards? What address have you given at work, or your pharmacy or your doctor? Which address is closer to your church or other significant social activities? Where do you sleep most nights?