Chuck here is a link to the DB150 page that tells you how yo use it as a booster and set it up.
http://www.digitrax.com/ftp/Empire%20Builder%20DT100IR.pdf
Still trying to make this simple for you, it seems that you would best benefit from having two power districts on the layout to make maximum use of the two power supplies, like having two blocks in DC. Your Zepher would power one block and the DB150 would power the other. Control of both blocks is accomplished from the one throttle through the LocoNet connections.
As I see it, what your concern is, is that the engines are drawing too much power for the one unit. That is taken care of by dividing the power requirements into power districts or blocks. All it take is to gap both rails, like you used to do to create the signal districts.
Adding the DB150 in line with the Zepher will not get you more power by adding the two together. Going back to a DC mentality for a minute, think about how you had to have a transformer for each loco and blocks. You never had two transformers connected to the same block except by all that complicated wiring and toggle switches. Now think of the boosters as a power pack that controls a block or power district, not an individual loco. The control of each loco within any block is done with the command station signals that are transmitted to both boosters at the same time.
Go back through some of the old postings here and you will find a lot of info that pertains to this present discussion too.
Let me know if any of this helps and try the Digitrax web site yourself foe Q&As.
www.digitrax.com
B-)
