I have two unused license. how to combine them to get 730 days license
That’s not possible, afaik.
Although avast could possibly do that, it will not be 730 days unless the licenses where bought today.
The period for a license start at the day you buy it.
so there is no way to get 730 days? 
Yes there is, buy a two year license.
Contact Avast:
http://support.avast.com/support/home
or
http://support.avast.com/support/tickets/new
not sure if they can still do this but they have for me before. the key is to contact them immediately after buying them. as stated the lic keys start ticking down from the day you bought them. so what you would end up with now is whatever days are left on each lic added together. as i said i do not know if they still can but they used to be able to do this a while back. next time i would just buy the 2 year lic’s to make life much easier.
Yes , or try to claim a refund of one of the 2 licenses to the support with explanations .