Under what circumstances would the OX and OM openly share  information/weapon/relic/tech?

I am drafting a campaign, and the simplest answer I came up with was: Not all Inquisitor's are shrouded in secrecy and distrust of one another.

Some are even friends. I know it seems like a stretch but that is why I am here =) To come up with a good reason.

Perhaps they expect to repaid, often immediately. 

Sometimes investigations into one thing can find yourself working with another Inquisitor / group of Acolytes from another Ordo because whatever you find might be related to investigations for both groups, and this is demonstrated in the Dark Heresy novels by Black Library.

In the second of those books, you have a Acolytes from Hereticus working with Xenos and Malleus because the people smuggling their lost rogue psykers are using Xenos weaponry and tech, and for whatever reason daemons (or what the acolytes believe are daemons) got involved because of the psykers.

And there's always a chance some poor Xenos (well, not poor as far as the Imperium is concerned) goes and gets themselves possessed, and the OX and OM might find themselves working towards the same goal.

Also, an Avatar of Khaine (from Eldar forces) is part-daemon, meaning the 2 groups might have to work together if a particularly large Eldar force is operating in the area (a large enough size to warrant the need for an Avatar to support the Eldar attack / defense).

I'm sure you could think of plenty of other reasons over time, but there's a few.

Humans are not the only races to allign themselves with the forces of chaos, in the Horus Heresy books there are plenty of xenos races the Marines fight who openly follow Chaos.

You could have them hunting a creature who at first is a suspected Xeno but turns out to be a daemon  (or the other way round) meaning the Acoyltes have to turn to Ordo Malleus/Xenos for help and opening conversation with the group.

I know the Ordo's are meant to be these big brotherhoods but each Inquisitor is a power unto himself, I'd say the reltionship between any two is like that of countries. Some get on really well while others detest each other, most the time there faction comes into play so a Thorian Malleus Inquisitor is most likely to work with a Thorian Xenos Inquisitor over an Istvavian Malleus Inquisitor.

good point, you've read your disciples =) 

I think I will go with the friends w benefits route... still plenty of time to work it out before Aug.