I have one camera (with one 3,5mm input) and two Sennheiser ew 100 ENG G3 wireless lavs. I need to connect these lav mic into my camera and I would like to record from each microphone into separate channel. How do I do that? Do I need a special cable?

Yes a special cable is needed.
Using a Beachtek unit solves many problems about getting signals into a 3.5mm input.

EDIT... One of the reasons to use a 'beachtek' type unit is it then gives you some sort of level control without going into the menus of the Sennheiser RX and it will also remove the 'plug in power' volts from the camera output, which have been known to damage some gear (like some brands / models) of radio mics.

Hosa Technology Stereo Mini (3.5mm) Male to 2 Mono Mini (3.5mm) Female Y-Cable - 6" YMM-261 .
Not to be confused with a stereo headphone 'Y' cable which looks similar.

Problem: However... using the 3.5 locking-to-3.5 (dual-mono) cable included with the G2/3 systems would probably short out. (Tip/ring tied together at destination end) So custom cables could be built or two standard mono 3.5mm cable could be used into the above adapter. The aforementioned 'Beachtek' device is a more expensive/ more gear option as well.

Is the camera 3.5mm socket designed for a stereo mic, or a mono mic?

Is the camera 3.5mm socket designed for a stereo mic, or a mono mic?

Good question. Since Mr. Berger withheld the identity of his camcorder, we can only assume it records two channels of audio. We also don't know whether we have to contend with plug-in power or not.

stereo mic, or a mono mic?
Right.. I just 'assumed' it would be a stereo input via a 3.5mmTRS mini .
assume= ass me

stereo mic, or a mono mic?
Right.. I just 'assumed' it would be a stereo input via a 3.5mmTRS mini .
assume= ass me

Since Mr. Berger asked about "Sennheiser ew 100 ENG G3 wireless lavs", he appears to mean two discrete mono mics.
Beachtek now makes that MCC-2 which seems custom-made for this application.
http://www.beachtek.com/products/hdslr/mcc-2-3/#

I have succesfully done this with my Canon t2i with a left right splitter.

Eric Toline at Audio Etc in Florida makes custom cables. He can make you one for this specific purpose.

audioetc@bellsouth.net

Please tell him I said hello.

Regards,

Ty Ford

I have succesfully done this with my Canon t2i with a left right splitter.

You mean ordinary left right splitter?
http://cdn-images.sewelldirect.com/products/SW-8433/SW-8433.jpg

My camera is an old Canon HV20 :) I has stereo input. I am not sure about the plugin power.

My camera is an old Canon HV20 :) I has stereo input. I am not sure about the plugin power.

Canon HV20 does NOT have plug-in power.

You can see the splitter I use at the bottom of my video. The first few frames. I used a T2i with this setup.

Dual Art Pre with Wireless V-Sound Microphone - YouTube

I'd be trying to use a preamp to allow separate gain control for each mic AND to uncouple the ground, which can lead to increased noise.

Every unbalanced 'stereo' input jack I've ever encountered had a common ground between L & R. Dropping the audio shield when tied to an A/C powered source device can in some instances eliminate a ground loop. A broadband 'buzzing' type noise is usually not ground related.