That is an interesting verse; (read the rest of the context starting in verse 10:26)
The specific sin in the OT that required two or more witneses was idolatry (Duet.17:2-7) The judgement for idolatry was death by stoning. If idolatry was punished by physical death - how much worse punishment should someone recieve who treats blood of Christ with disrepect? (in essence tramples on the Son of God) Counting his blood to be no more than a common thing - means to treat it as no different than the blood of an ordinary man or of that used in animal sacrifices. Or, by Insulting the spirit of Grace is refering to the Holy Spirit of God, an "agent" of God's gift of salvation. Any believer who commits these sins will be judged as worthy of a punishment worse than death.
Does this apply to the WT's view of Christ's blood and their teaching that it only makes atonement for 144,000? Or, do most of the members really reject Christ's blood by not partaking, which means counting it of no greater value? Possibly. And it could possibly apply to the WT in other ways too. Such as their belief and teaching that God's Holy Spirit directs the organization rather than directing all believers. Lilly