They are both Mag 3 and you need Mag 4 to hold a boss, so stacking them would work. Their duration on the other hand is shorter than controller single-target holds.
CoX 50s: <ill/rad> <ice/ice> <fire/kin> <grav/sonic> <ice/storm> <earth/kin> <kin/elec> <cold/psy> <thugs/dark> <fire/dark> <dark/elec> <night widow> <EM/ninj> <mind/icy>
Tesla cage can be slotted to overlap with itself for a second or two, as can Petrifying Gaze.
You should be able to perma-hold a boss using both of them, providing the Boss isn't very high level.
You can hold a LT but not a Boss with one application of either hold.
i think the main difference between controller and defender holds is duration, not sure but i think it's something like 85% for trollers and 75% for defenders (85% of what i'm not sure just saw those numbers somewhere)
Thanks for the information everyone, much appreciated. 
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk
Hi folks.
Im looking at my planned respec of my Dark/Elec Defender and I intend to include the two holds which I have access to, Petrifying Gaze and Tesla Cage. Now Im not used to magnitude stacking as Ive never tried playing a Controller so perhaps someone would be kind enough to tell me how effective Defender holds are and whether they will stack in the same way.
I seem to remember reading that PG is a Mag 3 hold but Im not sure on Tesla Cage. As its in my Secondary, I wondered whether its power would be less effective. Would stacking the pair hold a boss or are they two separate effects? Ill probably try the build out on test before going live but I thought Id ask here whether anyone has any experience of them in action.
Thanks.
@Dante EU - Union Roleplayer and Altisis Victim
The Militia: Union RP Supergroup - www.themilitia.org.uk