[Elecraft] ANTENNAS-ANTENNAS-ANTENNAS

[Joe Wiedeman]

There must be an answer:

I have two Cushcraft Model PC804N, 806 to 902mhz 4 element yagi antenna's.
I will be mounting them for vertical reception, one under the other on the
same mast.
I will be using a type "N" tee to couple the two antenna's to the feed
cable.

My questions are:
1:    What is the optimum spacing required between antenna's. ( the tips of
the longest elements )

2:    If each antenna has a gain of 4db what type of gain can I expect from
combining
      two of them this way. Will it double to 8db??

I remember the last time I posted a question and received answers to
everything but the basic question.
This one can't be too tough so please all you antenna mavens out there
please keep any answers in the
simplest manor and in inches not metric as I never got past second grade and
the knuckles on my fingers
scrape the ground when I walk.

PS
I have made several contacts on 20 and 18 for a total so far of seven hours
air time. I am running
my K2 at 100Watts using two 12ah gell cells in parallel. These were rag
chewing contacts (and I am long winded)
The batteries still read 12.85 volts during transmit.  I am wondering how
the non latching relays in the
automatic tuner is going to effect this performance. I may have to stick to
the MFJ 971 tuner when portable.
The batteries are INTERSTATE PC12100 12V 12Ah BSL1105
(interstatebatteries.com for anyone interested)
My best to all at Elecraft      K2MUF