Look at the Multifeed section of my website for some ideas on putting more than one satellite feed onto a dish
www.global-cm.net
Since you already have a 24-inch dish, I would install the LNBF for 82
with the rectangular tube attached to it at center. Then, looking from the front of the antenna, take the other LNBF (for 91) and go to the right and slightly lower; just wave it in the air off to the side of the previously mounted LNBF and look for the hot spot of signal. I have heard of people using a LEGO block as a spacer, and hose clamping around both LNBFs.
The trick is to get it stable. 24 inch antennas do not have the skew adjustment in back (in most cases) so you have to compensate for the elevation shift at the LNBF. Waving the 91 LNBF for best placement and then carving a wood block with a table saw to allow secure but accurate spacing...and then adding a hose clamp or two should do the trick...if you
are less than happy with the 2-LNBF adapter.
Mike