rad/elec

I dont have a high level character with this build, but Biospark was Rad/Elec (Level 24) before I remade him as an Empathy/Electric. And Way back at Launch I had the name Meltdown, who was also a Rad/Electric (Level 22 iirc) on my first game account.

It is a GREAT combo. Some tips I can think of off the top of my head are;

Take RI early and use it and RA as your main defenses till you get stamina.
EF is a great power, but unless you are only teaming, the endurance drain makes it not very useful solo (or anytime you try blasting a little) until you can slot up stamina and reduce the endurance cost of the toggle itself.

I personally dont have experience with Choking Cloud, but I would have skipped it myself. The last thing I wanted was another expensive toggle. RA, RI, EF, AM and LR seem like all the tools you would ever need solo or teamed. Everything else would be playstyle or extra goodies you just want to have.

On Teams, try to put RI and EF on different targets. Team members WILL defeat your anchors frequently, so its nice to have at least one running while you bring up a new anchor. On most of the teams I have joined with my RAD, I would always try and determine my team's usual habits so I could keep my auras running as much of the fight as possible. Usually these two toggles would end up on Harder targets like Bosses or on targets that people seemed to ignore longer and last longer during fights. I dont advise putting them on Minions, especially if you have alot of AoE, Blasters or Scrappers. Minions dont last long enough in those situations to be good targets for your anchors.

I strongly recommend checking out the guides section on Radiation for more tips.
There is alot of excellent advice on Radiation Emission there.

For Electric Blast, I have much more experience so here are some tips;
Take Tesla Cage at 28 ! Its very useful solo and on teams alike. Recycles fast enough to allow holding Bosses. Dont slot it as an attack, but as a hold (Hold, Acc, Rech).

You need to decide if you are going to use powerdraining as a defense or not.
To be honest, with Radiation as your primary, draining everyone's endurance may not be as necessary as it was for my Empath. If you decide to use draining, I highly recommend slotting SC for drain and getting it over 65%. I also recommend not trying to use drain as a tactic until at least level 22.

Electric blast has some flexibility, where you could stay back at range and blast when you need to, but I personally believe that the strength of Electricity is in Melee Range.
You should keep this in mind with regards to playstyle and how you plan on building your defenses. My Character's ultimate (read purpled-out) build will have 20%+ to all positions (Melee, AoE, Ranged and Psionic) due to the fact that I need to operate in close range alot. I could go with a Range-based character, but my melee/AoE would suffer.
With Radiation as your primary, being in melee range would be even safer, as you could put RI on a target (and assuming that target did not run off), you could colapse a mob around you for good old AoE blasting and overlapping protection of your toggles.

I hope this is what you were looking for, but I am sure there is a lot more knowledge out there for your combination.