First, confirm that your pedal is configured for stereo output.  For example, the Context comes configured for Mono In / Stereo Out, which is the right choice if it is the first stereo pedal in your chain.  If you have a stereo pedal in front of it, configure the Context for Stereo In / Stereo Out.  The Input/Output Configuration is set on the Config tab of the web editor (requires Google Chrome).

You will need a TRS to dual TS cable to split the left and right outputs to separate cables.  This article lists some examples:

The sides of the Y cable will connect to the two amps.  Since both of the amps are connected to ground through their power cables but also connected to the audio ground, there is a chance for a ground loop to develop.  The third prong of the amp power cables are connected to each other through the house electrical wiring and the input grounds of the amp are connected through the shields of the audio cables, creating a big loop that acts like an antenna.  It will sound like a low frequency hum or buzz (50 Hz or 60 Hz and harmonics):

To break the ground loop, you can use a transformer isolator or passive direct box with ground lift on one of the cables.  The other must be connected directly, so that it is grounded.  This breaks the loop while maintaining your signal:

Some examples of isolators or direct boxes are the Radial ProDI, Lehle P-ISO, and GigRig HumDinger.

The Y cables you use depend on the layout of your setup.  If the amps are close together, you may be able to use a TRS to dual TS insert cable and pull the TS plugs far enough apart to reach both amps.  For longer runs, a TRS male to dual TS female breakout cable will give you two 1/4" mono jacks to connect longer cables, but they do not always provide robust connections.  If space allows, you can connect a short Y cable to back of a small patchbay and run longer cables from the front of the patchbay to your amps.  Temple Audio has a 4-Way Jack Patch Module for their pedalboards that makes it very easy to connect.  Another option is to use two buffers to drive the long cables (place the isolator/direct box near one of the amps).