Repair joist. Acro prop while I do it?

[Albumen]

House built in 1927. Semi. 4 foot space under ground floor.

Just doing some tidying up. Notice a cracked joist. The timber looks completely sound, as does all the timber under the house. It's a sort of zig zag split from top to bottom. But there is lateral displacement, i.e. if I look at the joist from underneath the 2 halves of the joist are out of line, by a couple of millis. So I think the split goes all the way through.

I'll repair. What do I do? I'm guessing stainless steel coach screws to fix another joist the same size to the side of the existing? Or coach bolts and nuts? Do I need to prop anything while I do it to get the joist back up to level?

Thanks, Mike

 

[big-all]

picture will help
if its fully along and following the grain its unlikley to effect the strength

 

[foxhole]

Sister joist, no need to prop unless there has been significant movement.