From what I've read...
NO DON'T DO IT! Unless you have some means of
keeping them completely apart.
It seems they were bred for "silent fighting," which means there's calmness, even patience and
no grumble of warning 
before an attack. It states specifically there is
dog on dog agression! Goodness what would posess you to take on (give your word) such an animal with your own dog in the home already?
Does the owner have a
huge sturdy crate for him? Do you have a garage (safe from toxic fluids, etc.) where he can stay?
Crates for BOTH of them in separate rooms, maybe?
Sturdy walls and doors between them?
I would not, repeat NOT risk injury to either dog while you are away from home. I would also tend to limit their together time. I would exercise each separately. This is one of those situations where you may be kicking yourself in the head with remorse for having "given your word!"
