[Racal] RA6790/GM and diversity operations

[I Z 6 E G K]

Hi to the list,
I'm very interested in diversity reception mode, but I must to admit that I 
don't know so much about the matter.
Now I'm considering to get a second RA6790/GM, but there are some things I 
should like to know before...
Let's assume I have two antennas, let's say two big loops. Loop A is 
disposed orthogonally with respect to loop B.
Then I (will) have two receivers: one is master, connected to loop A and the 
other is slave, connected to loop B.
I'm reading on the RA6790/GM manual (page 2-6): "... For diversity 
combining, the Main IF AGC signal (on pin 21 of J3) of one receiver may be 
connected to pin 21 of the second receiver. ... The AGC circuits in both 
receivers will automatically respond to the proper signal.".
OK, but do I need any ancillary unit to link the two receivers together and 
make the things work or two rx and some wiring is enough?
Is the pin 21 the only connection needed between receivers?
Any enlightment is appreciated!
Many tnx and 73
Pierluigi IZ6EGK