Is it possible to have the interface of two or more context on the same vlan?
For example context 1 has gig0/1.1 on vlan 10 and context 2 has gig0/1.2 on vlan 10 as well.
Yes, contexts can use the same vlan. In fact this is a quite a common setup where the outside interfaces of the contexts are all in the same vlan and the dmz and inside interfaces are in separate vlans.
So you can have one IP subnet/vlan for all your outside interfaces.