I was filming the world kitsurfing championships in Coche Venezuela, when a fellow flyier told me that we couldn't fly at the same time due possible radio interference.Is that right?

I think I read 3-4 birds at the same time would be OK ... That coming from some one who works at DJI ... and that is probably the most they tried at the same time and found that there was no problem.
2 of them isn´t a problem any ways (we see that on you tube) !

It is not.

If they're both Inspires just open the radio preferences and choose a different channel.

If the other ones are FASST / signal jumping type controllers a la Futaba, they can just fly without issue (as is how they are designed). I've seen a dozen people with Futaba's all in the air together with no issues of any kind.

I was informed/ heard from another pilot that there was a reported issue with two Inspire pilots flying very close to each other and it resulted in one aircraft entering into the auto fail safe return home mode.  Once they moved apart about 20/30m there was no issue.

We see multiple Inspires flying together on you tube ...

In one video there was 4 of them ... flying very close!

Video feed might have interf ... but flight control should be OK.

It's not supposed to happen... Each Inspire 1 is paired with a different remote. It means that each remote has a different digital code paired with the inspire 1, so it will not accept the signal from a different remote.

Basically its save to fly, even if both of you are the same frequency

There might be problem if one of the Inspire has a weak signal. Its like 2 similar cars 2 differently coded remotes, and under special circumstances one remote can block the signal from a different remote. It will not open/close your car, but it will interfere with the signal, by blocking it.

Worst possible scenario I can imagine is: one of the inspires loses signal from the remote due to interference. I will start the RTH procedure and as soon as it gets closer to his paired remote, the signal will be back.

There should be no problem if you fly close to each other.