How to connect two mixers to work together

I have a Behringer 1002FX sound mixer so that I can set up the sound levels / EQ / FX independently of the main house mixer at my gig venues, and simply feed my 'processed' signal into the house main mixer driving the PA / speaker System.

Sometimes, maybe in the 2nd set of our gig, I have unexpected feedback...

Drives me nuts!! So I look on all the audio forums and read what I can about this issue... some say no problem; you can connect the Main Output from my Behringer into any Input channel on the main house mixer but "caution: not amplified output, if any"!! What does that mean?

Does this mean that the signal 'out' on the first mixer should not be processed (amplified / effected)? and that all amplification and sound control should be set / managed only on the second mixer?

Many threads on audio blogs and forums that I have read seem to differ...

Some have advised setting up the first mixer 'normally' with gain, EQ and level settings (as if it were the main mixer), then connecting the Main Out connectors on the first mixer to either a 2 X RCA connector channel on the second mixer or connecting each Main Out connector to each of two normal channel 'Line In' connectors on the second mixer? Just like I am doing... but an exception I saw was "the 1st mixer should send non-processed signals to the second mixer" so now I need to ask for advice on this!

Is there an issue with sending 'processed' (amplified, EQ'd and FX'd) signals on my 1st mixer to any 'Input' connector on the second mixer?

Thanks in advance...