Any suggestions on the above? How far apart should the antennas be? Which one on top?

Several feet.

Whichever works best in whichever position. I usually put the one on top that's having the most problems.

For some, a roof mounted tripod and mast is an excellent mounting option. Chimney mounts or eave/wall brackets may be a better option for others.

When mounting more than one antenna on a single mast, a 5' TV antenna mast is too short to permit adequate spacing between the antennas. I have used 1-3/8" diameter chain-link fence top rail for the mast in several installations. Last I looked, all are still standing and in great condition. If I have no compelling reason to do otherwise, I put the UHF antenna at the top. This aids UHF reception in most cases, and the wind loading is usually less for a UHF than a lager VHF antenna. I try to provide at least 3' of separation between antennas, though more can make a noticeable difference in many cases.

I've not used guy wire supported masts in quite a few years, but would still consider doing so if I needed to use multiple sections of mast.

Besides the local big box stores, Frys, Best Buy and Radio Shack stores, there are many online sources for antenna mounting hardware.

A few are,

http://www.3starinc.com/
http://www.amazon.com/
http://www.antennacraft.com/index.html
http://www.antennasdirect.com/
http://www.channelmasterstore.com/An...ories_s/39.htm
http://www.solidsignal.com/

and many that I've missed...

I'm already using a 10' mast with my current antenna. Will using a 15' mast with the DB8E and Y5713 and amp help enough to bother with or just stick with the 10' mast? I'm trying to avoid using guy wires.

I would hold off on the added height for now... I'd try the DB8E which will give you a significant increase in UHF gain. Only if the need is proven would I add a section of mast.

My concern is with both antennas on one mast the VHS antenna will be 4ft or so lower than it is now. ABC-10 and CBS-8 are pretty strong signals though.

VHF tends to bend (diffract) over terrain better than UHF so I start with the UHF on top. As ADTech already mentioned, you can try both ways to verify which is best at your location. And you're right to assume that the stronger and LOS path signals will be able to tolerate small deviations from the ideal antenna aim or location (better than weak 2-edge signals).

Which, in your case, is UHF on top.

My thought on who's on top is the Y5713 VHF antenna would offer some "bird protection" if placed on top. We have many large dove, hawk and owl. I'm not sure how durable the DB8E is as a bird post. Any thoughts?

My first thought, is that longer elements, will support less weight.

Many birds, will also avoid fishing line, strategically draped.

Unless you have condors, I wouldn't worry about birds and the DB8e.

Large birds don't go for small diameter perches. Birds, in general, will probably prefer the big VHF Yagi over a DB8e.

New antennas and amp are up and running. I've tried to adjust to recommended compass headings. My compass is all over the place due to metal in the roof, attic. I also rarely get full signal strength. Is there an best time of day to fine tune using the TVs signal strength meter? Do I tune to the strong or weakest station and then adjust for best reading?