It doesn't really matter, but as a point of interest, the opamp buffer-splitter you are interested in has nothing to do with the Tillman, which is a single JFET buffer. Also, note that it CALLS for a TL072. Yes, you can run the two buffers in parallel. For this taken to the Nth degree, have a look at
http://www.muzique.com/lab/superbuff.htm. As for the resistors, I doubt it will make much of a difference--it may increase noise very slightly, or cause more of a pop if you have a true bypass switch. . You could try it with the 3.3M and see if it works--if not, you can solder in another 3.3M or a 4.7M in parallel with the first 3.3 to get you closer to the original value. Alternatively, two 4.7Ms in parallel or two 1Ms in series would also be pretty close, but you could also probably go as low as 1M.
Edit: Antonis, great minds, etc. I would just add that paralleling the two opamp stages might be of value if you are using the buffer as a line driver, but in other applications it offers no particular benefit.
