Well I did it in 4 steps, but I think I can save you 2 steps which I suspect you had to perform before we got to where you passed us your example part, and if that's the case, then the net result will be the same:
4 steps - 2 steps = 2 steps
I say this because in your AVI showing the operation in Catia, I noted, as shown here...
...that it appeared that you created the two bodies using the Catia equivalent of the NX command 'Thicken Sheet', which indicates to me that these two bodies were originally sheet bodies (surfaces). Am I correct or not?
Look at the approach I took using NX 6.0, as seen in the attached part file.
Starting with the 2 sheet bodies, I performed 2 Trims, a Sew and then a Shell, for a total of 4 operations.
Let me know what you think of the approach which I took as compared to the OVERALL approach you took using Catia. I suspect that, if you're counting the steps needed, that we are basically at a wash here.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Cypress, CA
To an Engineer, the glass is twice as big as it needs to be.