mp10 paired with a mp40
 

I have a mp10 and a mp40,,,

which one should be the master? or does it matter...

what mode will create the most random flow,,,

and if the mp10 is at 100% what should the mp40 be at to create a equivalent flow

Does not matter which pump is the master. What kind of corals are you keeping? The type of coral can impact the mode you want to run in but it's really up to personal preference.

As stated, doesn't really matter. What does matter is that they are running the same firmware. Check your manual and set them up so they match. The one with the newest FW will update the older one wirelessly.
If you ever get a ReefLink you will always have the latest FW.

Before I bought a ReefLink, I preferred Nutrient Transport Mode (NTM) with Tidal Swell running.
After I bought a ReefLink, I setup all my pumps to run various modes throughout the day and night with different speeds. The online setup is really nice.
I swear by it now as I can program each pump individually (no more master/slave) and have full control over the 24H schedule.

This I can't answer as I've never run a 40, but math tells me a 10 runs about 35% of what a 40 can (at least by the new QD pump ratings anyway).

Without knowing what size and shape your tank is, your LR layout, and kind of corals/fish, it's hard to make suggestions.

With that said, if it were my 75g 4', I'd start off with the MP10 @ 80% and the MP40 @ 40-50%. Both would be in 'wave mode'. You go to Short Pulse (blue) and tune a wave. Once your wave is 'tuned' save it.
Then set your pumps to NTM Tidal Swell and see how it works over a few hours. It takes, IIRC, 1-1/2 hours to complete a cycle.
From this point you can 'fine tune' each pump's speed to your liking.

Hope this helps