I think you are just a little off. Either that or your players are just shooting themselves in the foot.
A kill-team may have more than 1 sustained power active at one time with the following limitations.
An Astartes may only sustain 1 sustained squad mode ability at a time.
An Astartes may only ever benefit from 1 sustained squad mode ability at any point.
An Astartes may only recieve 1 extra out-of-activation action each turn.
If an Astartes calls and sustains a sustained ability, then later joins in on an ability activated by another Astartes, or activates another sustained ability himself, the ability he first activated immediately expires. If he wishes to reactivate it he must first leave, or deactivate, the new ability and reactivate the other with full cohesion cost.
In your example above, the techmarine activated Strong Point and called his target. Other Astartes in the group then gain the benefit. Later the Devastator also activates Strong Point and called his target. The other Astartes in the kill-team must then decide to participate in the techmarine's or the devastator's sustained ability, they can not participate in both at the same time. If the Techmarine joins in on the devastator's ability, his sustained ability expires; but if he doesn't, the techmarine may continue to sustain his ability without penalty. So unless the Techmarine swithed to the devastator's ability, they would have only spent 6 cohesion and would have had two sustained abilities active.