Motion In RV Garage - Force Double Trip?

I have nearly 100 motion sensors attached to my Elk M1 but I'm only having issues with the motion sensor in my detached RV garage.  I keep my RV garage armed 24x7 (it's on a separate area) and on numerous occasions the alarm panel goes off due to my motion sensor.  One time I ran out there I saw a massive spider near the motion sensor.  I suspect my issue is related to bugs.
 
Is there anyway to make the motion sensor less sensitive... or make my Elk only trip the alarm if there are multiple hits of motion within x seconds/minutes?
 
Appreciate the advice! 

I think that could be done with rules.
 
Also, how about two motion sensors wired in series. Both sensors must be detecting motion in order to trigger an alarm. Not likely a spider could trip two motion sensors at the same time, while a person could easily do so.

Some other possible solutions:
1.) look for adjustments/dip switches on your existing motions, all the ones I have can be adjusted.
2.) switch to a dual tech sensor (infrared/microwave).

There is a cross-zone feature I recall - but it's been years since I've looked into it.  I'd search the manual to see if you can do it for just this one area without it affecting the rest of the house.
 
Otherwise you can do it via rules - but the trick is, there's no way to trigger alarm via rules only - you'd have to set two motions as non-alarm and also hook an output to an input and trigger that input as the alarm zone.  via rules, I'd either use a counter or flip a phantom output - then have it count down or reset after say 2-3 seconds.  If you get a trigger from the other sensor within that window, trigger the output that's tied to the input and trigger the alarm zone.
 
The above suggestion for two motions in series is a good and simpler option but requires each sensor to trip at the exact same time - and due to lockouts before resending it's quite possible to trigger one first, then the other - but not have them trigger simultaneously.

First, is your motion sensor designed for the temp range you are using it for?  With 100 motion sensors, cross-zoning is the way to go.  This is a feature that triggers an alarm only then two or more zones trigger within a set time, typically 2 or 3 minutes.  Also, don't you have sensors on the doors and windows? Maybe motion sensors are not needed?  Spiders on motion detectors do happen, that is why I enable cross-zonning for all motion sensors, but not on door ow window sensors.

Environmental would be the suspect here.
 
You didn't mention make/model, so part of the equation is the operating environment the detector is in, second being what technology the unit is using, followed by adjustment (if available) and whether or not the detector has any immunity involved.
 
Since you said this just started, you need to work backwards to what has changed in the environment since the installation. Is the detector sealed? Free from drafts, strong lights, or insects?
 
You need to address the detector and environment first before going the route of parallel installations or cross zoning, both of which are band-aids to what boils down to an installation issue.