Death feigning beetles make pretty good roommates for any number of other inverts, as long as they come from a similar environment and have basically the same requirements as far as temperature, humidity, substrate, etc. are concerned. They are popular "cage cleaners" in hairy desert scorpion cages because they can enter the burrows to clean up dead feeders or uneaten prey.
They may work, but I think you also may run into the issue that they both live in different environments. Orthoporus are kept a bit drier than other pedes, but the death feigning beetles need it very dry, perhaps with a small damp spot. Too wet and the beetles will get foot rot. If you put both species in a very large enclosure, you can have two separate habitats for them. Otherwise, I probably wouldn't do it.
As long as the millipedes molt underground there wouldn't be any problem. Even if they molt on the surface I highly doubt the beetles would try to eat them. And since only one or two people have ever successfully reared BDFs to adulthood you probably won't have to worry too much about larvae either.