How can I stop an Adult trike when both front brakes fail? I have a lovely easy adult trike and I use it all the time.
Now the thing is because it's a trike, it only has front brakes.

Well on the 14th Feb I was riding to my local charity, got to the hill and was 
riding it down when Twang the brake wire snapped and I desperately tried the other front 
brake this was gone too ... now this path leads on to a very busy road, with the two chicanes with metal fences ... with both brakes gone. I had no choice but to run into the second fence, a silver metal bar type.

Sorry about this long question is there any way I can add a brake to the two rear wheels of my trike to stop this happen again?

If this can't be done can I stop myself any other way?  .....

Update on this question. The brakes have been fixed for now... Tires are pumped up and I am oiling the Chain drive....Have to check the information provided.
Thanks please continue to answer the above question. Just bare in mind I am new...sorry if finding an answer takes a long time.. I like to look at all options.
Answering for completeness:

Don't get in this situation and Prevention is better than cure

You should have picked up that the brakes were not as good as they were by feel (poor braking) or on visual inspection noticed that the inner cables were starting to fray.

Most bikes have two independent brakes.  For yours to both fail at the same time is quite unlikely.

And even if you did successfully add a rear brake, a continuous lack of maintenance may leave that in as bad a state as the other two brakes,

A basic M check should be done weekly.