Yeah, I think a wee bit of scripting would be the best way to do it. C0LDSt33L's suggestion would almost work, except the player could drop the weapon, not "place" it, and it would still go off.
An event handler would do the job perfectly, though. Pop this in the player's init field:
bombslaid = 0; this addeventhandler ["fired", {if((_this select 1) == "put" && (_this select 4) == "pipebomb") then {bombslaid = bombslaid + 1}}]
That will make the variable "bombslaid" go up by 1 each time the player places a satchel. Then just make a trigger with condition:
bombslaid >= 2
And you're in business!