The two layers of 9 ounce doubled that you mentioned would be close to 1/4" thick if not over. The machine should be able to handle a size 207 thread, but that is the absolute heaviest it can handle. Your thickness of over 1/4" thick, which pretty heavy thread may be taxing the machine to its limits.
The 211 machines were good machines for upholstery and whatnot, but if you plan to sew heavier leather with thread in the 207 or larger range, then you might want to look at a heavy harness type machine, such as the Cowboy Model 4500, Cowboy Model 3500, Cobra Class 3 Cobra Class 4, and others.
Hope this is of some help to you.