well,there is no need to have a 8dbi antennas on an indoor devices.......I wonder if you have a really bad wireless signal in your house,as for most of our common users,5dbi is quite enough.

Thanks, friend.
Actually, i have one 5dbi and one 8db antenna from past devices...
Is it OK to mix them or must be 2x same antenna on router?

I think it's better use 2x same antenna on router rather than a 5dbi and a 8dbi.
I don't know if it will have a better performence in wireless coverage or not.But you can have a try,it won't do damage to your router if you use a 5dbi and a 8dbi mixed.