under simulation settings check all the additional cloth collision options which are:

vert against cloth

poly against cloth

cloth self collision (maybe don't need this one if they aren't moving too far)

 

even at the lowest settings for collision offset and collision depth, i don't get pokethrough when i check these

however at rendertime, smooth polygons might give you some poke.  you'll have to raise the collision offset for that, unless you uncheck smooth polygons in render settings.

finally, if any collision object crosses another collision object sqeezing the cloth between, you'll get holes in the object appearing sometimes all over the object.  or sometimes the sim will just fail.  even a single vertex crossing another vertex across the cloth surface will do this.  so elbows, shoulders, hips, knees, or the two people lying next to each other can make this happen.  make sure the cloth doesn't get between these polys.  it doesn't look like it from here, but i realize this is a single frame from an animation.  maybe someone turned and pushed the cloth between a body part and the bed?

 

go that way really fast.
if something gets in your way
turn