Hi Mafort,
Please let me know (either here or via PM) how it goes with the flattening?
I'm in a very similar phase to you. I split my gemsbok horn, soaked it in water for a couple of days and then tried both steam and dry heat to flatten each half between two hardwood boards.
Unfortunately both halves barely gave any movement and simply cracked down the "crown" of the halves. As a result I'm now forced to quarter the horn into thin strips and am going to simply sand them flat before sticking two quarters on each limb lying parallel with each other. I'm still working out how I'm going to deal with the two mini crowns from each quarter. I'm probably going to just sand them to reduce each crown a little bit, but I don't want to lose too much thickness.
As bownarra pointed out both here and to me in the previous post, it's clearly best to stick the horn on a convex core. That's what I'm going to do for my next attempt, because I clearly am going to have zero luck from steaming.