I will try the second shop vac connection if the single doesn't work well enough and the coal doesn't freeze. Since my pipe run is slightly downhill from the bottom of the outside bin to the top of the inside barrel I don't have the "head" issue to overcome when lifting the coal from the basement floor level so I am hoping the single vac will work over the long term. What are the hp ratings of the vac's you are using? I have a 6.0 peak hp vac on there now.
I don't want to go through the effort of tapping another hole in the top of my barrel for the second vac if the coal is going to freeze up in the outside bin making my vac system useless. The barrel I am using does not have a removable lid and has 15 gal capacity so I can't reach from the bottom port to the top in order to thread the fittings. I need somebody with orangutan arms to reach in there for me!
I added a couple vents in the ends of the bin so air could flow across the top of the coal. This was more to keep the moisture from the coal pile condensing on the underside of the roof and rotting it out than to dry the coal out, but it may help do that also.