With Radiation you will always need stamina. Because of the great end usage of your debuffs.
The endurance gained in the elec attacks is barely noticeable, so don't count on it to replace Stamina.
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>
Oh well, was too good to be true, I guess the synergy will be at least decent with rad/elec then.
You will get other sets side effects from your primary already (-def (Rad), -recharge (Psi), -acc (Dark) and -rez (Sonic)). So end drain will be a good bonus for you IMO.
I know its been answered but would like to add a Hell No, but short cicuirt followed by EMP should leave mobs with no end and held so I like the sound of the combo
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
I'm currently working on one of these and i must say it works a treat in the role of enemy neutralisation. Not only can you start throwing around debuffs to cripple large chunks of the enemy mobs but paired with Choking Cloud and Short Circuit you can get in among the mob and nuke their end and hold some of them as well. You will need stamina though or slot rest so you can use it after every fight...
You may find yourself a little short on damage but thats what teamates are for.
Works a treat on those Mastermind types too 
The pellet with the poisons in the Vessel with the Pessel, the Chalice from the Palace has the brew that is true...
My musings from my level 50 Rad/Elec defender:
Synergy:
Actually nothing spectacular. Short Circuit and EMP have a slight synergy (end drain and -recovery) but firing off both will NOT end drain everyone. It will hold them for an awfully long time.
This combination works best as a melee defender. Of course, you cant always do this when lots of AoE mezzing gets flung at you because if your debuffs die, you tend to aswell.
The most effective combination is. Lay down the holy rad trinity (Enervating feild, Lingering Rad, Radiation Infection) and jump in. Everyone clusters around you (and hopefully the tank). All enemies in a nice neat debuff radius. CHoking cloud kicks in, as a fantastic hold. You unleash short circuit. 2 applications of this will end drain everything to zero.
Put all this together, and you may not be suprised to hear that you can actually "tank" mobs as long as you dont get mezz'd.
Acrobatics for kb protection is useful. Getting flung out of a tightly packed group is annoying at best, disasterous at worst.
A few other observations:
Rad Must Haves: Enervating feild, Lingering Rad, Radiation INfection, Choking cloud.
Rad Goods: Everything else. (And yes, I include fallout particularly if you want to do the STF but I would get it late for AV hunting)
Elec Must Haves: Short Circuit. Do. Not. Miss.
Elec Goods: Telsa Cage
Elec Miss: Thunderous Blast. Do. Not. Get. You have no endurance = enemies have no debuffs = calamity.
Stamina is NOT skippable. The elec endurance buff (to you) is small. Short Circuit on a large mob is about the only time you will notice a net end gain from a power. THe endurance buff, at best, should be thought of as something that allows you to blast without huge end problems.
Overall I think Rad/Elec is a very effective build for neutralizing enemies. It debuffs everything, end drains, and has very effective holds in primary and secondary. In addition, ELec offers at least two energy damage AoE effects which is a nice damage boost for a team.
Overall I prefer to it Rad/Rad. With the rad primary enemy defence is debuffed quite amply, and the extra effect of your rad secondary (-defence) is generally academic unless you are fighting high level AV's.
Mmmm, you guys gave me a lot to think about since both choking cloud and fallout were out of my build, but hey, I'm still lvl 5 and things can be amended. This toon is meant for an all defender team, but I'll keep in mind soloing / other AT teaming options just in case.
Thx
Bear in mind that with 2 Kins on the team (I'm assuming neither of them have changed!), endurance shouldn't be a problem so you'll have plenty of room for all the lovely AoE effects! Also, mez protection should be pretty much covered between us.
Well, I managed to squeeze Fitness although that means I'll pass on Leadership (maybe Maneuvers instead of Acrobatics if I manage to get shiny unique KB prot enhcs). I am also skipping Thunderous Blast hence I'll be able to put Choking Cloud. Fallout could be considered at lvl 47 - 49 for STF which I think we should try with an all defender team, at least for a few laughs.-
To be honest, even with all the toggles I'd be surprised if any of us needed Stamina after 2 applications of Speed Boost. I'm definitely planning on skipping it, but that may change if the structure of the group changes.
Eagerly awaiting new Sonic/ animations though - it might finally look like I'm doing something! Those who play with the sound on will already be aware of this, however 
Heya,
I just rolled a Rad/Elec defender. Basically, I wanted to know a couple of things, first, if this combo has a good or at least decent synergy and second, if I read correctly, some of the endurance drained from elec attacks is added to my own endurance, hence I'd like to know if this will be enough to skip Stamina (no sets considered) or if I will stil need Stamina.-
Thanks.-