if one house has 49 seats and the other 50 out of 51 what happen next?

The vice president casts the deciding vote , and Democrats WIN again ! YAAAAAYYYYY!

the senate can't have 49 and 50 seats............

its 100 not 99

50 > 49.     

Not sure what your question is.

If you are talking about houses of the same legislature, the answer is nothing.  Most legislatures have a larger lower house and a smaller upper house.  But each house is considered equal, and bills have to pass both houses.

If you are talking about parties in a particular house, then it depends on the rules for that body.  For example, in the U.S. Senate, the party with the most votes chooses committee chairs and usually controls the floor schedule (i.e. which bills get debated on the floor).  But it takes sixty votes to end debate, so a 51-49 majority is not enough to pass bills.  

If you are talking about a vacancy in a particularly legislative house, there are rules about how the vacancy are filled.  Until the vacancy is filled, the rest of the house continues to function with only the seats that have been filled.  In the U.S. House, the state governor is required to call a special election to fill the vacancy but state law determines when that special election will be.  In the U.S. Senate, the governor can appoint somebody to fill the vacancy and that person will serve until a special election is held (with state law determining the scheduling of the special election which might not be until the next general election as we have seen in Georgia).

Then the side with 50 gets 50 votes, and the side with 49 gets 49 votes until that last member is seated. The same thing would happen if it were 50-50 and a senator dies in office. Until a replacement is appointed by the governor, they function with 99 members.