As you are aware home breaker pannels are powered by two-120v legs of power running into it. Also known as "phases". Phases "A" and "B". Make sure you are running on the same phase. I'm not framiliar with the ds7000 but i'm assuming of course, that it's line controled? Make sure whatever outlets you have them plugged into are on the same "Leg" of the line-in to your breaker pannel. Every-other breaker going down the panel, will be on the same leg. On both sides of the panel. Like so.
Line A
l Line B
l l
[] A [] <------ DS7000
[] B [] <------ SIREN
[] A []
[] B []
etc.
This first setup would be problematic without a phase coupler; or with a deffective/improperly installed coupler.
The second one should work because the DS7000 and the siren are running on the same 120v leg of the 240v line in.
Line A
l Line B
l l
[] A [] <------ DS7000
[] B []
[] A [] <------ Siren
[] B []
If this second setup works, your phase coupler is either installed incorrectly, or it's deffective.
Incase you couldn't tell [] represents a breaker in the panel. lol
Anyway, hope this helps.
Also, I'm still relatively new to X10, though i've got a few years of electrician experiance under my belt. Like I said, i'm not framiliar with the DS7000 but the theory is pretty universal. If somehow any of this is incorrect, someone please correct me!